China already has a zero-tariff policy for imports from 33 African countries, but Beijing said last year it would extend the policy to all 53 of its diplomatic partners on the continent.
China is Africa's largest trading partner and a key backer of major infrastructure projects in the region through its vast "Belt and Road" initiative.
From May 1, zero levies will apply to all African countries except Eswatini, which maintains diplomatic relations with Taiwan.
China claims the democratic island as its own and does not rule out using force to take it.
Many African countries are increasingly looking to China and other trading partners since US President Donald Trump imposed steep tariffs worldwide last year.
Xi said the zero-tariff deal "will undoubtedly provide new opportunities for African development", announcing the date as leaders across the continent gathered in Ethiopia for the annual African Union summit.
