The multiple longitudinal mode(MLM) laser sources can be depolarized by a crystal waveplate with moderate thickness.For two available MLM LDs,the relationship between DOP of depolarized light and optical path difference(OPD),which is produced by the waveplate between two orthogonal oplarized components of incident light,is numerically calculated.An experimental depolarizer installation is proposed in which OPD can be adjusted continuously in a large range.The theoretical analyses and experimental results show that there is the most appropriate thickness of waveplate for a specific MLM laser source,and this thickness is decided by center wavelength and the space between two longitudinal modes.Moreover,DOP becomes lower if the symmetry of spectrum is well and the number of relative large longitudinal mode increases.