Abstract: To study the PM_2.5 changes after implementation of the clean air action plan in the Yangtze River Delta region, an emissions inventory for the Yangtze River Delta region in 2012 was developed, and the emissions reductions during 2013-2017 were evaluated based on measures described in the clean air action plan for Shanghai, Jiangsu, Zhejiang and Anhui provinces. The WRF-CMAQ modeling system was applied to analyze the PM_2.5 changes under the emission reduction scenarios. The clean air action plan is related to six areas including energy, industry and transportation, etc. Under the three emission reduction scenarios, including strong, medium, and weak measures, the total SO₂ emissions will be reduced by 74.5×10⁴,3.8×10⁴, and 34.4×10⁴ t, respectively; NO_x will be reduced by 108.7×10⁴,3.9×10⁴ and 61.1×10⁴ t; primary PM_2.5 will be reduced by 40.3×10⁴,6.1×10⁴ and 14.6×10⁴ t; and VOCs will be reduced by 98.2×10⁴,7.0×10⁴ and 23.5×10⁴ t. Modeling assessment shows that under the weak, medium and strong emission reduction scenarios, the annual average PM_2.5 concentrations at the national observational monitoring sites within the Yangtze River Delta region will decrease by (4.4±1.1), (8.1±2.4) and (12.5±3.9) μg/m³, respectively. Compared to the base year, the average PM_2.5 concentration in the Yangtze River Delta region will be reduced by 8.7%±2.2%, 15.9%±4.7% and 24.3%±7.7% on average. The results show that significant emission reductions and new source controls are required to attain the 2017 air quality improvement target.