The modulate factor is an important parameter in analog FM station. The instantaneous frequency of the FM signal is the result of module factor and FM signal working together, and the module factor can be calculated by instantaneous frequency. But the calculation precision of instantaneous frequency is easily affected by the noise of the received signal. When the SNR(Signal Noise Ratio) is higher than 20dB, the relative error between modulate factor and real value will be lower than 1%. The common ways to calculate instantaneous frequency include:time-frequency analysis; short time Fourier transformation and so on. The calculation precision is easily affected by the factors of cross term, window choices and noise. Besides, because of using two-dimension transformation, the calculated amount is larger and the property of real-time is not so good. So a new algorithm of particle filtering is given in this paper, which can reduce the noise and operation amount and improve the precision of estimation. The simulation experiment verifies the efficiency of the algorithm.