Water vapor plays an important role in the earth-atmosphere system, and the change of water vapor is an important reference for the change of weather and climate. Raman lidar has outstanding performance and is widely used in water vapor detection. It is very important to determine the calibration constant term in the inversion of water vapor mixing ratio detected by Raman lidar. It is common to use the radiosonde data to determine the calibration constant. However, the cost of simultaneous release of sounding balloons is high, and the radiosonde data of the global radiosonde network is limited in time and space. Compared with radiosonde data, ERA5 reanalysis data has higher spatiotemporal resolution and wider coverage. Many scholars analyze and apply ERA5 reanalysis data, which verifies that ERA5 reanalysis data is in good agreement with radiosonde data. Therefore, this paper proposes to use ERA5 reanalysis data to determine the calibration constant at the place and time without radiosonde data. The calibration constant of water vapor inversion with Raman lidar is determined by ERA5 water vapor data in Hefei. In the altitude range of 0.5?2 km, the calibration constant changes with time and altitude, which is considered to be caused by data deviation, interpolation calculation and lidar system error. The relative error between the inversion results and ERA5 data is mostly within ±10% by substituting the mean profile of the adjacent time calibration constants.