Based on available data and theoretical models, the computer simulation of laser radar for direct detection is demonstrated. The basis of the theory model of laser radar for direct detection is built on that the maximal detectable distance is decided by certain detection probability, false-alarm rate, signal noise ratio and threshold noise ratio. In this simulation, it is supposed that the detector noise be denoted by Gaussian probability intensity function, the absorption and scattering of atmosphere and aerosol be interpreted by the U S Standard Atmosphere Model. The simulation results show that this simulation software of direct-detection lidar can give reasonable data under various practical conditions.