Measurement analysis of atmospheric aerosol with APD detector was described.Aatmospheric particles went through laser beam which was scattered.The scattering light of particles through two parallel laser beams was detected by an APD (avalanche photoelectric diode) detector,by forming a double-peaks signal,and the fly-time of the particles was obtained.The instrument was calibrated with standard particles.It can be used to measure the size distribution of atmospheric aerosol particles.The standard spectrum was obtained by analyzing the experimental data of different size particles,and measured the atmospheric aerosol particles and standard particles.The experiment results agreed with the theory very well.