The high-power microwave (HPM) short pulse radar uses high power microwave as transmitting signal, which doesn't work well with the model of the conventional radar transmitting signal. This paper analyses the characteristics of HPM signal, and finds that it has irregular envelope and jittery phase. Therefore, we fit its envelope by Gauss multi-peak model and its phase by polynomial model. We obtain its pulse modulation model, and verify the method of reconstruction with small residual by Matlab. Our study is significant to the simulation of this new kind of radar.