SHANGHAI — Faced with mounting American tariffs that could slow China’s already weakened economy, President Xi Jinping is pressing the case to the rest of the world that China can be a positive force for global trade.

The challenge will be convincing the world he means it.

Mr. Xi, the country’s top leader, on Monday kicked off a six-day expo intended to show China as a big buyer of foreign goods. He extolled China’s appetite for what the rest of the world makes, and he reiterated promises that Beijing would lower import tariffs and improve intellectual property protection to help make that happen.

“Openness has become a trademark of China,” Mr. Xi said. “China has grown by embracing the world, and the world has also benefited by China’s opening up.”

“Today, as economic globalization deepens and develops,” he added, “the ideas of the law of the jungle and winner take all increasingly lead to a dead end.”