Depolarizer plays an important role in realizing Raman pump source with low degree of polarization(DOP) and in solving the problem of polarization dependent gain(PDG).According to the optical spectrum of Febry-Perot(FP) laser,the performance of the depolarizer for multiple longitudinal mode laser is analyzed in frequency domain.Simulation results show that:1) The FWHM of the spectrum has an effect on the performance of a depolarizer.2) The spectrum width of every longitudinal mode has no effect on the performance.The conclusion is that crystal length depends on the mode space and envelope of the spectrum.A good estimation for designing the best crystal length with 5% DOP is presented.Crystal length will save one-third for depolarizing the common used FP laser.