The 28 northern cities were ordered to cut concentrations of small, breathable particles known as PM2.5 by 10-25 percent during October 2017-March 2018. They have been curbing industrial output, thinning traffic and cracking down on coal use in a bid to limit smog build-ups.
The Ministry of Environmental Protection said that all the cities exceeded their targets in the final three months of 2017.
But the ministry, citing meteorologists, said unfavorable weather, including relatively warm temperatures, is expected to make smog dispersal more difficult in January and February.
The six-month campaign in northern China had appeared to start slowly, with only four of the 28 cities meeting their October targets.
But an inspection and enforcement campaign began to have a significant impact in November.