Prices at the wholesale level edged up only 1.8 percent in 1985, contributing to the nation's smallest three-year inflation rate in 20 years, the Labor Department reported yesterday.

The small increase in the producer price index last month nearly matched the 1.7 percent rise in 1984, with the help of restrained food and energy prices most of the year. For 1983, the rise was only 0.6 percent.

Food prices, which had climbed 3.5 percent in 1984, dropped most of last year before rising sharply in the last three months, Labor said. Consumer energy prices fell 4.1 percent in 1984 and were virtually at the same level at the end of 1985 as at the end of the previous year.

In December, producer prices rose 0.4 percent following increases of 0.8 percent in November and 0.9 percent in October. The slowdown last month was because of smaller increases in food and energy costs, Labor said.

President Reagan has made reducing inflation one of the major goals of his administration. Prices at the consumer level rose 3.2 percent last year through November, following a 4 percent rate of increase for all of 1984.

At the White House, presidential spokesman Larry Speakes said yesterday that the producer price report showed "a remarkable record of low inflation."

Many private economists said they don't expect a reacceleration of inflation this year and that, after temporary increases in the past three months, increases in food and energy prices should moderate.

"You're going to see food prices in 1986 grow much more slowly than they have been in the last couple of months," said Lawrence Chimerine, chief economist for Chase Econometrics. "It will not be a factor in inflation in 1986.

"If oil prices keep softening, as we think they will, we'll probably see crude refined product prices softening in the spring," Chimerine said. "You're going to get these couple of months of blips" in some areas such as food and energy. "They are basic interruptions in a basic trend of flat prices in these areas."

Economists have said that a possible influence on inflation could be the decline in the value of the dollar, which has fallen more than 20 percent since last February. A decline in the dollar's value tends to make imports more expensive. Higher-priced imports also tend to remove pressures on domestic companies to keep prices low.

However, economists said that although some companies, notably Japanese consumer electronics and automobile concerns, have raised prices, inflation has not taken off and is not expected to for the next 12 months.

The government measures price increases at three stages of production: crude, or raw, materials; intermediate, and finished goods. The price changes in finished goods, reflected in the PPI, generally are passed along to consumers several months later.

Although the increases in food and energy costs were somewhat volatile last year, the index excluding those two components rose a modest 2.7 percent, following a 2.1 percent increase in 1984, suggesting that there are no other areas likely to accelerate inflation.

Prices at the crude level fell 5.5 percent during 1985, the largest annual decline since 1952, the Labor Department said. The index for intermediate goods also fell 0.3 percent last year, following a 1.3 percent increase in 1984.

In December, prices for finished consumer goods rose 0.5 percent following a 1.0 percent rise in November. The index for consumer foods increased 0.8 percent compared with a 1.6 percent rise in November.

Prices for beef, veal, pork and processed poultry fell "after rising considerably in both preceding months," Labor said. After jumping 12.6 percent in November, fish prices rose more slowly, Labor said.