WASHINGTON—President Trump ramped up his attacks on China and the world trading system, threatening Friday to have the U.S. unilaterally revoke the special breaks global rules grant to nations that call themselves developing countries.

The move, coming on the eve of a new round of U.S.-China trade talks in Shanghai, appears aimed at pressuring Beijing to commit to new specific measures to buy more U.S. goods and further liberalize its market for foreign companies.

“The...