And that could be a "real danger," Baucus told CNBC.
U.S. and Chinese officials met this week in Washington to talk about trade. During those meetings, an agreement between the two countries including possible tariff reductions started to take shape, a senior administration official told CNBC on Friday.
The summit between Kim and Trump is set to take place in Singapore, but it's unclear if it will occur. Kim has threatened to pull out and on Wednesday Trump said "we'll have to see" if the summit is still on.
On Thursday, Trump said the North Korean leader was possibly being influenced by Beijing.
"It's very dangerous because we don't know what's in Kim Jong Un's mind. We don't know what's in [Chinese President] Xi Jinping's mind. We don't know what's in Trump's mind," said Baucus, a Democratic former senator from Montana.
"We can't trust Kim. It's also a bit hard to trust Xi Jinping. So I wouldn't give up a lot of stuff now hoping we can get Kim to go to Singapore and get a deal," he added.
Baucus also slammed Trump's push to get Chinese company ZTE back into business in the U.S. Last month, the Trump administration barred U.S. companies from selling to ZTE for seven years. The ban was in response to the company's shipping of American goods to Iran and North Korea in violation of sanctions. ZTE has said the ban was unacceptable and threatened its survival.
While top administration officials tried to separate Trump's comments from the trade talks, Trump muddled that message earlier this week. He said on Twitter that his reversal only relates to a "larger trade deal" his administration seeks with Beijing.
Baucus said that while the trade issues with China are legitimate, ZTE should be prosecuted for violating the law.
The White House didn't immediately respond to a request for comment.
— CNBC's Jacob Pramuk and Reuters contributed to this report.