In view of the difficulties in retrieving Digital Terrain Model (DTM) with the spaceborne lidar data, the terrain elevation estimation accuracy of Ice, Cloud, and land Elevation Satellite-2 (ICESat-2)/Advanced Terrain Advanced topographic laser altimeter system (ATLAS) strong and weak beam data under the forest was studied, and the effect of canopy height and vegetation coverage on ICESat-2/ATLAS estimation accuracy of DTM was explored. The results show that the accuracy of DTM under the forest with strong beam estimation accuracy is R²=1, RMSE=0.74 m, and that with weak beam estimation accuracy is R²=1, RMSE=0.76 m. The performance of the strong beam estimation accuracy performed better than that of the weak beam, but both the photon cloud data of the strong beam and the weak beam can provide scientific data for estimating the DTM under the forest. In the study area, with the increase of the canopy height and vegetation coverage, the error of different laser types data gradually increases.