Xi's remarks on Taiwan, the democratically governed island claimed by Beijing, came in a closed-door meeting of the leaders of the world's two largest economies that ran more than two hours.
They represented a stark - if not unprecedented - warning during a pomp-filled occasion that was otherwise friendly and relaxed, although the US summary of the talks made no mention of Taiwan.
Instead it focused on the leaders' shared desire to reopen the key waterway of the Strait of Hormuz, effectively closed due to the Iran war, and Xi's apparent interest in buying American oil to reduce China's dependence on Middle East supplies.
With Trump's approval ratings dented by a war with Iran that shows no signs of abating, the first visit by a US president to China in nearly a decade has taken on added significance as he searches for economic wins.
"There are those who say this may be the biggest summit ever," Trump told Xi on Thursday after a welcome ceremony at Beijing's Great Hall of the People.
Xi told Trump that preparatory negotiations between US and Chinese economic and trade teams had reached "balanced and positive outcomes", China's foreign ministry said.
The talks aimed to maintain a fragile trade truce struck when the leaders last met in October 2025, where Trump suspended triple-digit tariffs on Chinese goods and Xi backed away from choking global supplies of vital rare earths.
Treasury Secretary Scott Bessent said he expected progress on establishing mechanisms to support future bilateral trade and investment, and an announcement about large Chinese orders for Boeing aircraft.
Trump expected Xi to raise the thorny issue of US arms sales to Taiwan, he said before flying to China.
With the status of a $US14-billion ($A19 billion) package awaiting Trump's approval still unclear, China has reiterated its strong opposition to the sales.
The Chinese leader told Trump that Taiwan was the most important issue they faced and if handled poorly could push the entire US-China relationship into an extremely dangerous situation and even cause the countries to enter conflict, according to Beijing's summary of the talks.
Trump did not respond to a reporter's shouted question whether the leaders had discussed Taiwan as he posed with Xi later for photos at the Temple of Heaven.
Taipei said there was nothing surprising from the summit and that China's military pressure was the real threat to peace.
At a lavish state banquet on Thursday evening attended by senior officials and business executives, Xi told the audience that the China-US relationship was the most important in the world.
"We must make it work and never mess it up," Xi said.
Joining Trump on his visit are a group of CEOs looking to resolve issues with China, from Elon Musk to Nvidia CEO Jensen Huang.
Trump entered the talks with a weakened hand.
US courts have hemmed in his ability to levy tariffs at will on exports from China and other countries, while the Iran war has boosted inflation at home and escalated the risk that Trump's Republican Party will lose control of Congress in November's midterm elections.
Though the Chinese economy has faltered, Xi does not face comparable economic or political pressure.
Aside from trade, Trump is expected to encourage China to convince Iran to make a deal with Washington to end the conflict, as a fifth of global supplies of oil and natural gas travel through the Strait of Hormuz in normal times.
But analysts doubt Xi will be willing to push Tehran hard or end support for its military, given Iran's value to Beijing as a strategic counterweight to the United States.
