A three-dimensional amplifying model for the laser system is presented. Both free space transportion model and ray-tracing amplification model are included in this model. Using the model,the distribution of output laser beam from a complicated amplifying system close to the reality can be obtained. This model is used to simulate an amplification course in a 12-pass laser diode-pumped amplifying system. Simulation results precisely show the output distribution of the laser beam from the 12-pass amplifying system. Based on the simulation results,the 12-pass beamline can be further optimized.