We analyze and experimentally verify the impact of facet reflectivity on slope efficiency and output power of 450-nm GaN-based semiconductor lasers. The results reveal that for asymmetric resonator structures, the nonlinear effect of longitude spatial hole burning can be suppressed by optimizing the facet reflectivity, thereby improving the differential quantum efficiency and maximum output power of the device. A high slope efficiency of >1.3 W·A -1 is obtained at the facet reflectivity of 5%, and a high power output of 2.6 W is obtained at the operating current of 3 A.