That equates to 9.47 million barrels per day (bpd), down 11 percent from April, which has one more day, Reuters calculations showed, pressured by lower imports from Iran and as several major state refineries were shut for regular overhauls.
"The main reason for the fall in China's crude imports is the Iranian imports which fell sharply in May," said Seng Yick Tee, analyst at Beijing-based consultancy SIA Energy.
The world's largest oil importer started to sharply scale-back imports of Iranian crude in May in the face of tougher US sanctions on Tehran's oil sales.
China has instead been stepping up purchases from other Middle East suppliers such as Saudi Arabia, Iraq and the United Arab Emirates, as well as from Brazil.
Still, China's crude imports in the first five months of 2019 hit a total of 205 million tonnes, up 7.6pc from the same period last year.
Meanwhile, China's oil products exports also slowed in May, to 4.49 million tonnes, down from 6.17 million tonnes in April, the customs data showed.
Oil product exports for the first five months hit 27.09 million tonnes, up from 25.55 million tonnes in the same period in 2018.