This study uses ECMWF fifth-generation reanalysis,ERA5,which extends to the mesopause,to construct the Initial Conditions (IC) for WACCM (Whole Atmosphere Community Climate Model) simulations.Because the biases between ERA5 and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature data are within ±5 K below the lower mesosphere,ERA5 reanalysis is used to construct IC in the lower atmosphere.Four experiments are performed to simulate a Stratospheric Sudden Warming (SSW) event from 5 to 15 February 2016.The simulation using the WACCM default climatic IC cannot represent the sharp meteorological variation during SSW.In contrast,the 0～4 d forecast results driven by ERA5-constructed IC is consistent with ERA5 reanalysis below the middle mesosphere.Comparing with WACCM climatology ICs scheme,the ICs constructing method based on ERA5 reanalysis can obtain 67％,40％,22％,4％ and 6％ reduction of temperature forecast RMSE at 10 hPa,1 hPa,0.1 hPa,0.01 hPa and 0.001 hPa respectively.However,such improvement is not shown in the lower thermosphere.