If Taiwan is "mishandled," it could lead to conflict between the two countries, Xi said.
It would put the entire relationship between China and the United States in an extremely dangerous situation, he said.
The US president will visit China in two days - his first trip since 2017 - and is expected to discuss both Iran and trade policy with his Chinese counterpart.
Around 4 p.m. Swedish time, Trump was welcomed at a ceremony outside the Great Hall of the People in Beijing.
A Chinese military band played the countries' national anthems, and cannons were fired. Schoolchildren waved the countries' flags.
"We should be partners and not rivals," Xi said at the ceremony, and he received support from his American counterpart, Donald Trump.
"It is an honor to be your friend, and the relationship between China and the United States will be better than ever before."
The leaders plan to hold talks on Thursday and Friday, with several items on the agenda. During the visit, Trump and his delegation will attend a banquet and visit a temple, among other things.
Defense Secretary Pete Hegseth, Secretary of State Marco Rubio, and a number of American corporate executives will also participate in the state visit.
Trump and Xi began their bilateral meeting behind closed doors immediately after the ceremony, according to the BBC.
