Linear Frequency Modulation (LFM) interference mitigation in received GPS signal by antenna array was studied. The pivotal problem of LFM interference suppression is the instantaneous frequency estimation, With this conception, the cross-term in the time-frequency distribution must be suppressed. Thus the received signal vector was whitened and the Spatial-Time-Frequency Distribution (STFD) matrix was formed by utilizing the whitened signal vector. A decision variable which is different from traditional one was deduced. By using the decision variable, the auto-term in the time-frequency distribution of the LFM signal can be selected clearly. Based on the time-frequency distribution of the auto-terms, the parameters and the instantaneous frequency of the LFM signals can be estimated. According to the instantaneous frequency, corresponding notch filters can be formed, which can mitigate the interferences. Through simulation, it can be viewed that the time-frequency distribution of the received signal which is interfered seriously by the cross-term can be mapped into a clear time-frequency distribution of auto-terms of the signal. Under the condition that the number of snap is enough, the LFM interferences can be suppressed clearly.