This report describes the experimental researches on the polarization Oharaoteristios of symmetric GaAs-GaAlAsP double heterojunotion lasers, and analyses the mode-competition processes of these lasers. The experiments showed that semiconductor laser is emitted spontaneously and does not indicate optical polarization characteristic when it is biased under the threshold current. When it is biased above the threshold current, the laser for thin active layer of d = 0.15?0.40枓m ig generally observed only in fundamental order mode, and TE mode polarization is predominant. At this time, polarization selection is dependent on Fabrj-Perot cavity facet (cleaved face) mode reflectivity E0. But TM mode is saturated at the threshold, the onrrent applied to the laser above the -fclixesliold is used to enhance the TE polarization when the active thiokneas d is larger than 0.4^m, the competition, between TE and TM mode, fundamental order and higher order mode will appear. TE mode enhancement will result in TM mode decay, or TE mode decay provides TM mod? enhancement. In this case, polarization selection depends on both the mode confinement factor Γ and facet reflectivity Rm. TE mode, TM mode, fundamental-order mode and higher-order mode will possibly appear. Besides, polarization charaoteristio depends closely upon the internal crystal defect and inhomogeneous crystallization for the aotive layer of laser as well as the partition ratio of injected carrier.