Based on a dry deposition model, we evaluated the PM_2.5 particles reduction effect of plain afforestation project in Beijing. The results showed that 451 t PM_2.5 particles had been reduced by the trees planted in Beijing plain areas in 2012. Among these trees, broadleaf forest contributed 57% of the total reduction. The reduction effect was variable in different seasons, with 50% of reduction happening in spring. The plain afforestation project had made the annual average PM_2.5 concentration in the afforestation area decreased by 0.57 μg·m^-3 that has achieved 2% of the target from “Beijing Clean Air Action Plan (2013-2017)”. The plain afforestation has played an important role in the urban pollution management. Considering that there is no effective way to control the PM_2.5 pollution, we recommend continuing carrying out the plain afforestation project in appropriate circumstances.