Fine particle pollution is a major public health concern in China. This paper assesses China’s potential population exposure to PM2.5 at the subdistrict level, the smallest census unit with public information. Using both ground-based measurements and satellite-based Aerosol Optical Depth retrievals, we estimate subdistrict-level daily PM2.5 concentrations by means of the block cokriging approach. Our results reveal that China’s population-weighted annual average PM2.5 concentration during April 08, 2013 and April 07, 2014 is nearly 7 times the annual average level suggested by the World Health Organization (WHO). About 1,322 million people, or 98.6% of China’s total population, are exposed to PM2.5 at levels above WHO’s daily guideline for longer than half a year. We also simulate the effects of China’s most recent action plan on air pollution control. The simulation results demonstrate that the population exposed to PM2.5 above China’s daily standard for longer than half a year will be reduced by 85% if the plan can achieve its target by 2017. Nevertheless, PM2.5 will be still at a harmful level to public health, which calls for additional effort in pollution control.