In this paper, a new method of demodulation of continuous phase binary FSK signal is presented, during FSK demodulation, the FSK signal produces In-phase signal (I(t)) and Quadrature signal (Q(t)). And method of Gardner synchronization is used for bit synchronization. When using Gardner synchronization, some changes must be made so that it is suitable for FSK demodulation. It can resist 7% of frequency deviation. And it gets lower bit error rate than traditional method of coherent demodulation when SNR is the same. What’s more, there is only 0.1dB SNR loss, 9.78 dB to 9.88 dB, when the bit error rate is 1e-3 when we compare demodulation with Gardner synchronization to optimal receiver demodulation.