The principle of frequency discrimination(FD) by digital transversal filter is discussed. A detailed analysis of the FD method by autocorrelation and auto-convolution is given and the slope and bandwidth of the characteristics of FD is obtained. In addition，the curves ∑I～ωand⊿Q～ω are computed and given in this article. This FD method can be used for AFC of the coherent-on-receive system.