For the performance analysis of the domestic spaceborne laser altimeter in the laboratory and in-orbit calibration technology improvement, the principle of laser waveform full-link simulation was studied in detail, and a full-link simulation method with the fine terrain and flow were designed by considering the in-orbit working environment of the spaceborne laser altimeter, as well as its echo simulation model of was deduced in detail. Then high-precision echo simulation was performed by laser beam splitting based on fine terrain. The Geoscience Laser Altimeter System (GLAS) is used for testing, and the high-precision airborne Lidar point cloud data is used to simulate the waveform of GLAS in flat and mountain terrains. The results show that the simulated waveform are used compared with the real data from GLAS, then the similarity between the simulation waveform and the real waveform is 0.985 in the flat terrain, as well as 0.921 in the mountain terrain. The proposed method and model can achieves high accuracy simulation of the spaceborne laser waveform for the flat areas.