Based on Marius’s law in the“Physics of University Physics”，the polarization characteristics of GaN-based semiconductor lasers are studied accurately by self-built experimental platform. The experimental results show that after the lasing threshold，the GaN-based semiconductor laser has a higher degree of polarization and it is close to 1. At 5 mA far below the lasing threshold the polarization degree is close to 0.70，and it is already close to 0.93 at 27 mA below the threshold. This shows that not only the spontaneous emission with no degree of polarization but also the linearly polarized light with the same polarization direction as the threshold exists in the light emission of the GaN-based semiconductor laser far below the lasing threshold，and the ratio of this linearly polarized light in the total light intensity is very high.