Abstract: Recently, the research of micro-Doppler effect provides a new approach for accurate target identification, which possess an important application prospect of helicopter detection, classification and recognition. In this paper, the radar scattering cross section was predicted with the physical optics method, and micro-Doppler model of a helicopter rotor blade was modified. Based on the model, the echo signal of double blades and three blades with different shape and rotating speed was calculated and micro-Doppler effect was analyzed with STFT. The results show that different quantity, height and rotating speed have different influence on the curve shape, amplitude and period of the micro-Doppler, and some reference for helicopter identification by micro-Doppler is provided.