CMAQ v5.1 and CMAQ v4.7.1 were applied to simulating the air quality in Yangtze Ｒiver Delta (YRD) region in January， April， July and October in 2015 ( representing winter， spring， summer and autumn respectively) ， and the simulation results of regular pollutants ( NO2， SO2， O3 and PM2.5 ) were compared． The results indicated CMAQv5.1 showed a better simulation performance of NO2， SO2 and PM2.5 concentrations than CMAQv4.7.1， but worse simulation performance of O3 concentration． The simulated NO2 concentrations of CMAQv5.1 were closer to the observed values than that of CMAQv4.7.1． The deviations of NO2 simulations were more affected by the overestimation of emissions instead of chemical mechanisms of different versions． No obvious difference in SO2 between the two versions was observed and both of simulated SO2 concentrations of two versions were close to the observed values． The simulated PM2.5 concentrations of CMAQv5.1 were closer to the observed values．The improvement on PM2.5 simulation was obvious， attributable to the newly added secondary organic aerosols and the emission subdivision species in the updated mechanisms． The simulated O3 concentrations of CMAQv5.1 were higher than that of CMAQv4.7.1．During the period with high O3 concentration level， the simulated O3 concentrations of CMAQv5.1 were closer to the observed values． However， the simulated O3 concentrations of CMAQv4.7.1 were closer to the observed values during the period with low O3 concentration level． CMAQv5.1 performs better than CMAQv4.7.1 during the periods when observed Maximum Daily 8-hour Average ( MDA8 ) O3 concentrations exceed the standard， while CMAQv4.7.1 preforms better during the periods when concentrations do not exceed the standard． In general，the O3 simulation has not been effectively improved． It is recommended that，in the future，the modification of O3 simulation in relatively low-level period and region will be helpful for the improvement of O3 simulation．

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