A bumper crop of opium from Southeast Asia's Golden Triangle area after two years of drought threatens to glut the American and European heroin markets later this year, according to U.S. and Thai narcotics officials here.

Heroin produced from the opium crop has already begun to hit the streets here, and the number of Americans and other foreigners arrested here on smuggling charges is on the rise, officials say. Those caught face harsh prison sentences under a new Thai law, with jail terms for major offenses ranging between 25 years and life.

Despite their efforts to stop the traffic, narcotics officials expect about 90 percent of the Golden Triangle heroin to elude them. As a result, about 27 tons of high qulity heroin from the area straddling Burma, Thailand and Laos are expected to flood Southeast Asia and the rest of the world.

So far, very little if any of the Golden Triangle heroin has reached the United States, American narcotics officials say. But they regard an influx as virtually inevitable, and that could mean serious problems for cities such as New York, Los Angeles and Washington.

"The prospects look pretty bad," said Dick Hart, the head of the U.S. Embassy's Narcotics Control Unit here. He said the number of heroin addicts in the United States had been steadily declining since the late 1960s, when the figure reached about 600,000.

By the end of last June, the number had dropped to about 380,000. But it started to rise again toward the end of the year. Now officials estimate there are as many as 450,000 U.S. addicts.

Based on past experience, narcotics officials expect the influx of Southeast Asian heroin to be accompanied by further increases in the number of U.S. addicts and in the purity of the drugs sold. Heroin prices may drop somewhat, and more overdose deaths and emergency room admissions can be expected. In addition, some U.S. cities could see higher crime rates and new outbreaks of gang warfare, officials say.

Despite expectations of increased work here, the U.S. government apparently is going to reduce the staff of the Drug Enforcement Administration's regional office here, transferring that of the last two years -- seems likely to surpass production from the Golden Crescent region that includes parts of Iran, Afghanistan and Pakistan, according to the DEA office here.

Disruptions caused by resistance to the Soviet occupation of Afghanistan and by the Iranian-Iraqi war are believed to be limiting the Golden Crescent's opium production this year to 450 to 650 tons, down from 1,300 tons last year. By contrast, DEA officials here forecast that the Golden Triangle's crop "may just exceed 650 tons."

The DEA expects about half of this amount to be converted into heroin, yielding about 35 tons of the drug. Of the amount that eludes seizure, the DEA says, about 70 percent is usually consumed in Southeast Asia, 23 to 25 percent is smuggled to Europe, Australia and New Zealand, and 5 to 7 percent makes its way to North America.

Last year, according to DEA figures, Americans used 4.5 tons of heroin, about a third of it from Southeast Asia. Because of the anticipated short-fall from Southwest Asia this year, narcotics officials expect the percentage from this area to increase.

"We expect the Golden Triangle to become the number one threat to the United States and Europe," said DEA regional director Robert De Fauw. "We'll be witnessing a glut in the heroin market in late summer, fall and winter this year and early spring next year."

Largely responsible for the anticipated surge in the heroin trade, De Fauw said, is an elusive Burmese warlord and trafficker named Chang Chi-fu. fHaving expanded his operations along the Thai-Burmese border during the last six months, Chang Chi-fu, 48, now ranks as the undisputed king of the Golden Triangle.

Although most U.S. addicts probably have never heard of him, drug enforcement officials consider Chang Chi-fu the world's biggest narcotics dealer. De Fauw calls him "enemy number one."

According to DEA intelligence reports, Chang Chi-fu controls 70 percent of the nine to 15 heroin refineries currently operating just across Thailand's northern border with Burma. The laboratories are located along about 100 miles of the border in rough, mountainous terrain covered by thick jungle canopy. They are virtually inaccessible from the Burmese side, and Thai authorities have refrained from going after them in Burmese territory.

Chang Chi-fu reportedly lives about 300 yards inside Thai territory near the village of Ban Hin Taek not far from where the borders of the three Golden Triangle countries meet. Although he is wanted by both Burmese and Thai authorities, he has been able to slip back and forth across the border to avoid arrest.

Chang Chi-fu also has a formidable force at his disposal. The titular head of a Burmese separatist group called the Shan United Army, he is backed by 3,500 to 5,000 guerrillas who have managed to acquire modern weaponry.

In an effort to restrict the narcotics traffic, the DEA has been urging the Thai and Burmese governments to launch a joint operation against Chang Chi-fu.

However, Thai authorities apparently are not promising anything.

Thai police Maj. Gen Pow Sarasin, chief of the Office of the Narcotics Control Board, said he hoped a joint Thai-Burmese raid would "take place some day."

He said the Thai Air Force last year bombed five depots on the Thai side of the border used to store chemicals for heroin production. Western sources said, however, that the raids did little damage.

Pow also expressed criticism of plans to cut back the DEA's presence here.

As proof of the Thai government's commitment to suppress narcotics, authorities point to a tough 1979 law providing for life imprisonment or execution for producing heroin, smuggling it in or out of the country and selling or possessing more than 100 grams.

One of the growing number of victims of the new law is John Hamilton Fuller, 31, formerly of the Washington area.

According to Thai police reports, Fuller was arrested Jan. 27 at the Bangkok international airport and charged with trying to smuggle out 425 grams of high-quality heroin.

Interviewed recently at the Medical Correctional Institute of Bangkok's Klong Prem Central Prison, Fuller, looking fit, said he was "pleading guilty to everything" during his trial.

"I had to come here to support my habit," Fuller said. He said he had been an addict for seven years before he was caught and twice underwent methadone treatment in Arlington.

After his arrest, he said, it took him six weeks to get off heroin, without methadone or any other drug. He said that during his ordeal one of the nurses used to tell him, "Better you die."

Seemingly resigned to his fate, Fuller said he hoped to get a minimum sentence of 25 years.