Simulation modeling and analysis of laser radar echo signal is an important research content of ladar imaging,detection and target recognition. A novel way based on coordinate transform is proposed to solve the light travel distance of object scene by studying the mathematical model of airborne scanning ladar laser echo. A method of solving the light travel distance based on a coordinate change is proposed. The simulation computation only depends on the number of laser spot segmentation in the simulation, thus has large advantages in simulation modeling of complex target scene. Based on these methods, the Z scanning airborne ladar laser echo in complex target scene with shelter is simulated, and the experimental result is obtained. The simulation datas indicate that laser echo not only has the information of target distance, but also has target surface type information and surface reflectivity information. The results provide the foundation in future study on detection of obscure target based on the ladar echo.