Major drug syndicates in Southeast Asia have opened a new transit route through China to facilitate smuggling of heroin from the "Golden Triangle" to lucrative markets here and the West, according to narcotics officers.

The so-called "China connection" was inaugurated about three years ago by drug traffickers looking for easy land passage to Hong Kong from the Golden Triangle region of Burma, Thailand and Laos, where most of Asia's opium is grown, said police and customs agents.

Couriers started a new trail of heroin, a distillate of opium, by slipping across the laxly policed border of Yunnan Province straddling the opium-rich region. The drugs move overland to Canton, where they are loaded onto fishing trawlers or railway cars destined for this British colony and its 40,000 to 50,000 heroin addicts, authorities said.

Most of the heroin remains here but some is sent to the West Coast of the United States, according to police sources.

Other drug smugglers have taken advantage of lax customs inspections at Chinese airports, said police. Posing as businessmen or tourists from Southeast Asia, they fly to Peking or Canton with concealed shipments of heroin, then board Chinese airline flights to the United States, Europe or Hong Kong.

Although the smuggling operations are financed and organized in Hong Kong and Bangkok, they apparently are assisted by local Chinese, according to police.

The Chinese connection, which became possible after Peking opened its borders to trade and tourism, still is regarded as a lesser transit route from the infamous Golden Triangle. But drug specialists here said it provides traffickers greater flexibility in escaping detection.

"It's another card in their pack," said Hong Kong's commissioner of customs and excise, Douglas Jordan.

"The idea is to send the stuff through two or three stops and to increase the number of couriers to throw us off," explained C.J.W. Bagley, acting chief narcotics officer for the police department.

China's government originally denied that its territory was being used as a transit route for narcotics, reflecting official sensitivity left over from the nation's once widespread opium addiction.

But after prodding from Hong Kong, Chinese authorities finally acknowledged the problem in 1981 and began training customs agents to search for opium and heroin entering China from the Golden Triangle.

A special narcotics staff was set up in Canton, which is a center for trafficking because of its proximity to Hong Kong and its international air links. Canton police now have telephone lines to their Hong Kong counterparts. Chinese authorities have visited here to learn laboratory analysis of hard drugs.

Jing Bing, head of China's observer delegation at the U.N. Commission on Narcotic Drugs, told a session in February that foreign traffickers have "taken advantage of China's open-door policy" to smuggle drugs through China to other countries. In July, Chinese customs announced intentions to take "resolute antismuggling measures."

The crackdown actually began about two years ago. According to Chinese statistics, custom agents in Peking and Canton tracked down 18 cases of heroin smuggling from early 1981 to the end of 1982. They seized over 132 pounds of the drug and arrested 36 traffickers, who were later sentenced to prison terms ranging from five years to life.

In the first eight months of this year, Hong Kong police and customs agents cracked five cases of drug smuggling from the Golden Triangle through China.

Jordan said cooperation from China has aided enforcement here. Although intelligence agents in Bangkok often can learn the destination of narcotics, they cannot always trace their paths.

Earlier this year, customs agents arrested a Hong Kong man coming from China with 27.5 pounds of opium from the Golden Triangle. With the help of Canton police, agents were able to uncover friends and relatives in Hong Kong who were involved in the smuggling network.

Despite some recent progress in enforcement, police despair of ending drug smuggling across the long and rugged Yunnan border, which is said to be as difficult to police as the U.S. border with Mexico.

"How can you stretch a security net across all of Yunnan Province?" asked Bagley.