To attain high spectrum resolution.

A heterodyne beat signal is filtered by at least one matching filter in this optical spectrum analyzing method and this optical spectrum analyzing system of heterodyne base. In one embodiment, an input signal 302 and a sweeping local oscillator signal 306 are combined in an optical coupler 310 to be output to a receiver, so as to generate the heterodyne beat signal. The heterodyne beat signal is filtered by a matched filter unit 350, and the filtered heterodyne signal is used to generate an output signal for expressing an optical parameter of an input signal. The heterodyne beat signal exhibits secondary phase behavior in a laser of high interference. An S/N ratio and the spectrum resolution are enhanced remarkably by utilizing the secondary phase behavior of the heterodyne beat signal in the signal processing.