In order to detect and recognize rotors based on laser micro-doppler effect, the modulation of laser echo by rotating area rotor is studied. Micro-doppler echo model of rectangular area rotor is built using the physical optics method of scattering echo superposition. Simulation of laser echo based on this model is completed and the micro-doppler feature different from line target rotor is extracted using time-frequency analysis method. Mechanism analysis confirms that the feature contains lateral size information of rotor. This paper proposes a method to calculate the chord of rectangular rotor through laser echo and verifies this method to be effective through rotor echo simulation of various aspect ratio. Error analysis shows that using the method of changing the window size of short-time Fourier transform (STFT) to extract time and frequency information respectively can reduce the chord length calculation error to 1.58% compared to single size. This model can realize the calculation of aircraft rotor chord size, providing an important reference for the research of rotor identification.