China will restrict imports of scrap steel and aluminium from July 1, the environment ministry said on Saturday.
Trump, citing national security concerns, placed tariffs of 25 percent on steel imports and 10 percent on aluminum imports.
May will appeal for freer and fairer trade and launch a defence of the rules-based international system at this week's meeting of G7.
The prospect of a trade war poses "downside risks" to the US economy, which otherwise is poised to grow at a solid pace.
U.S. President Donald Trump set import tariffs on Thursday of 25 percent on steel and 10 percent on aluminum, to come into force in 15 days.