Imports of Russian oil rose by 55% from a year earlier to a record level in May, displacing Saudi Arabia as China's biggest provider.
China has ramped up purchases of Russian oil despite demand dampened by Covid curbs and a slowing economy.
In February, China and Russia declared their friendship had "no limits".
And Chinese companies, including state refining giant Sinopec and state-run Zhenhua Oil, have increased their purchases of Russian crude in recent months after being offered heavy discounts as buyers in Europe and the US shunned Russian energy in line with sanctions over its war on Ukraine.
The imports into China, which include supplies pumped through the East Siberia Pacific Ocean pipeline and shipments by sea, totalled nearly 8.42m tonnes last month, according to data from the Chinese General Administration of Customs.
That pushed Saudi Arabia - formerly China's biggest source of crude oil - into second place with 7.82m tonnes.
In March, the US and UK said they would ban Russian oil, while the European Union has been working towards ending its reliance on Russian gas, as the West steps up the economic response to the invasion of Ukraine.