Chinese leaders have indicated to Sen. Henry Jackson (D-Wash.) that they are unhappy with a delay in promised U.S. trade benefits and are beginning to wonder what they have gained from normalization of relations between the two countries.

The critical remarks by the Chinese apparently including Vice Premier Deng Xiaoping, come just before Saturday's scheduled arrival here of Vice President Mondale. The remarks appear to signal an effort by the Chinese to win concessions or assurances from Mondale on the trade issue, which the Chinese see as vital to their economic future.

While refusing to quote any Chinese leaders directly, Jackson said today after a 2 1/2-hour meeting with Deng yesterday that "the longer the delay [on approving most-favored-nation benefits for China] the greater the danger of an adverse impact on our relations" with China.

To underline what he sees as the great value of stronger ties with Peking, Jackson described a new Chinese oil find off Hainan Island. Jackson said Chinese oil experts told him that a new well in that area, thought to have great potential, had struck good quantities of "low sulphur crude," the most marketable variety of petroleum.

The Carter administration reached final agreement with the Chinese in July on a trade agreement that would lower duties on some Chinese goods as much as 50 percent or more. But some members of the administration and Congress have favored delaying submission of the bill for congressional approval in hopes of arranging for similar benefits to be extended to the Soviet Union.

This action, Jackson said, has produced in China "a general feeling in the leadership, a state of unhappiness as compared to what was anticipated after normalization," the creation of full diplomatic relations between Peking and Washington earlier this year.

Asked at a press conference here what the Chinese would do about the delay, Jackson said, "The Chinese do not talk in retaliatory terms: but added "I believe it's bad for us that we will be delayed in attaining a stronger relationship with China, which I feel is crucial to the peace of the world."

Mondale is reportedly scheduled to see both Deng and Communist Party Chairman Hua Guofeng, who received Jackson today. Besides trade, Mondale is also expected to discuss the Chinese refugee problem, visit Sian and perhaps reopen for the first time in 30 years an American consulate in Guangzhou (Canton) before leaving for Hong Kong Sept. 1. There have been reports from Washington that President Carter would visit China early next year.

A 1974 trade act amendment co-authored by Jackson barred most-favored-nation trade benefits for any nation that did not provide assurances it was taking steps toward free emigration. Since last year the number of Chinese being allowed to leave the country to reunite with relatives in the United States has climbed from about 30 to, at one point, 3,000 a month, and is much higher for Chinese going to Hong Kong. Emigration from the Soviet Union has also increased substantially, but while the Chinese have given Washington their official assurance that they are relaxing bars to emigration, the Soviets have refused to do so.

Some members of Congress and the administration have discussed accepting the increased Soviet emigration as a tacit assurance of a new emigration policy but Jackson, an outspoken foe of Moscow and strong supporter of Peking in Congress, said today he opposes that.

"You'd have to change the law in order to do that," he said. "The Soviet Union has not complied with the law." But in China's case, he said, the Carter administration has been "negligent" in not submitting the trade agreement to Congress.

The Chinese have demonstrated their support for Jackson, on his third trip here, by arranging meetings for him with both Deng and Hua, an unusual gesture to someone who is not a head of state or close to it. Jackson also was taken to the Sino-Soviet border in the Xinjiang region. He was told of an August violation of Chinese territory that he would not comment on, other than to say it involved no shooting.

Jackson said the Chinese told him they were ready to handle provocations from either the Soviets or the Vietnamese. He said that in case of an full-scale Soviet attach, the Chinese said they could field 300 million guerrilla fighters in the form of their national militia.