The operating principle and simulation results for the photon counting chirped amplitude modulation（AM）ladar using Geiger Mode avalanche photodiode（Gm-APD）detector are introduced.The key parameters of the system and the recovery of the modulating signal are studied and simulated.The simulation result shows that,for our system to detect a 2 km target,the minimal emitting optical power is 6.4 W.It also shows that the wider the pulse width outputted by the Gm-APD is,the smaller the IF signal is,which is opposite to the SNR.On the other hand,the photon-counting chirp AM ladar has a good ability to control the modulating distortion and distinguish multi-target.