A routine air quality data assimilation (DA) system was established at the China National Environmental Monitoring Center (CNEMC) based on the optimal interpolation (OI) method. The surface observations from more than 1,400 stations across China were assimilated into a real-time air quality forecast system with three nested domains. The initial conditions of NO2, SO2 and PM2.5 in the three domains were optimized by the data assimilation system. The impact of the data assimilation on the real-time PM2.5 forecast over the Beijing-Tianjin-Hebei (BTH) Region during the heavy haze season of 2015 was evaluated. The results show that the DA can significantly improve real-time PM2.5 forecasts, reducing the root mean square error (RMSE) by 23%, 8.2% and 4.8% in the forecasts of the first, second and third day, respectively. The mean fractional bias and the mean fractional error of the forecast were reduced from 50.9% and 70.67% to 40% and 62.3%, respectively, and the performance changed from “criteria” to approaching “goal” (as defined by Boylan and Russell, 2006). Additionally, increasing the assimilation frequency can improve the DA system performance for real-time forecasts. As can be seen from the various cases studied here, the improvement in data assimilation is more significant when the bias of the model is higher and there is still much room for correction. The results also show a rapid decay of the DA effects on the PM2.5 forecast, which highlights the limitations of the current routine data assimilation system in which only initial conditions are optimized. Further improvements in the data assimilation system with meteorological data assimilation and chemical parameter optimization are needed.