China's central bank conducted 140 billion yuan (21.5 billion U.S. dollars) of seven-day reverse repos and another 130 billion yuan of 14-day reverse repos on Friday.
Offset by 90 billion yuan of maturing reverse repos, the operations resulted in a 180-billion-yuan net injection into the money market.
For the whole week, the net injection totaled 40 billion yuan, in contrast with a net 510 billion yuan withdrawal last week.
The central bank had suspended open market operations for 12 work days prior to Wednesday, citing sufficient liquidity in the banking system.
A reverse repo is a process by which the central bank purchases securities from commercial banks through bidding, with an agreement to sell them back in the future.
The PBOC said earlier that it would conduct open market operations in a flexible way to meet the liquidity needs of banks.
China will continue a prudent and neutral monetary policy in 2018 as the world's second-largest economy strives to balance growth with risk prevention.