One of the crucial components of an X-ray focusing lens's performance test is effective area calibration. The focusing lens will be calibrated on the ground before being launched into space. The X-ray source can't achieve the ideal condition of an incident on the mirror in parallel due to the distance restriction of the ground experimental device, so its calibration results will be different from those in orbit. In order to understand and correct the discrepancy between the effective area of the Wolter-Ⅰ X-ray focusing lens in ground calibration and in-orbit calibration, the effective area of the X-ray telescope carried by the Einstein probe was simulated and analyzed under two calibration conditions. The ground calibration experiment of effective area was carried out in the 100-meter X-ray calibration device of the Institute of High Energy Physics, Chinese Academy of Sciences.First, prior to designing the light source, the calibration model is created, then the incident conditions of on-orbit and ground light sources are taken into consideration. The program makes use of X-ray sources with energies of 1.49 keV, 4.5 keV, and 8.5 keV. Second, the light source used for the on-orbit calibration can be thought of as a parallel light source while the light source used for the ground calibration can be considered a point source. The effective area of the focusing lens under the two calibrations is determined using the two incident light sources to mimic the ground calibration and the on-orbit calibration, respectively. The deviation percentage of their effective areas is determined using the simulation data. To determine the connection between the effective area and the off-axis angle, the off-axis analysis of the focusing lens's effective area is performed within 1°, and the change diagram of the off-axis effective area is drawn. Lastly, the ground calibration experiment is run in the Institute of High Energy's 100-meter calibration device. The effective area of the focusing lens is determined using the photon count on the monitor before and after the addition of the focusing lens in the experiment, which employs continuous spectrum X-rays. The useful area is corrected based on the simulation-derived deviation ratio. In a similar manner, the off-axis experiment is conducted within 1, and the change diagram of the experiment's off-axis effective area is made.The findings indicate that when the X-ray energy is 1.49 keV, 4.5 keV, and 8.5 keV correspondingly, the effective area of ground calibration is 2.7%, 3.0%, and 4.0%, bigger than that of on-orbit calibration. The difference between them increases with increasing X-ray energy. This phenomenon is compatible with the energy-dependent decrease in X-ray reflectivity. In the simulation and experiment, the off-axis angle affects the focusing mirror's effective area, which falls by 20% when the off-axis angle approaches 10'. The research presented in this paper demonstrates that the effective area of ground calibration is always greater than the effective area of on-orbit calibration and that the effective area of on-orbit calibration of the focusing lens can be predicted using ground calibration data combined with simulation. The off-axis effective area of the focusing lens also demonstrates a clear relationship with the off-axis angle. This study's methodology can offer data references for the Wolter-Ⅰ focusing mirror-equipped X-ray astronomy satellite's effective area calibration.