Theoretical analysis has been made of several kinds of Raman pump laser depolarization based on Jones matrix and Stock parameters and the numerical computation of the depolarization degrees of the actual laser spectrums performed.When the Polarization Beam Combined（PBC） method is adopted

the minimum Degree Of Polarization（DOP） can be obtained only when the optical powers of the two lasers are identical.However

when PBC plus the birefringent crystal depolarization are adopted

the DOP after PBC is always smaller with the variations of the crystal length than that when a single laser runs through the crystal.Therefore

the crystal length should be so designed as required by single laser depolarization to maintain the DOP with different pump sources.The design method and conclusion presented in this paper may contribute to the depolarization design for the pump laser of Raman amplifiers.