WASHINGTON — When President Trump welcomes President Xi Jinping of China to his palm-fringed Florida club for two days of meetings on Thursday, the studied informality of the gathering will bear the handiwork of two people: China’s ambassador to Washington and Mr. Trump’s son-in-law, Jared Kushner.

The Chinese ambassador, Cui Tiankai, has established a busy back channel to Mr. Kushner, according to several officials briefed on the relationship. The two men agreed on the club, Mar-a-Lago, as the site for the meeting, and the ambassador even sent Mr. Kushner drafts of a joint statement that China and the United States could issue afterward.

Mr. Kushner’s central role reflects not only the peculiar nature of this first meeting between Mr. Trump and Mr. Xi, but also of the broader relationship between the United States and China in the early days of the Trump administration. It is at once highly personal and bluntly transactional — a strategy that carries significant risks, experts said, given the economic and security issues that already divide the countries.

While Chinese officials have found Mr. Trump a bewildering figure with a penchant for inflammatory statements, they have come to at least one clear judgment: In Mr. Trump’s Washington, his son-in-law is the man to know.