Using lasers to achieve high beam quality and high peak power output are research hotspots in recent years. This paper adopts the method of matching the refractive index of the core layer and the cladding layer and uses the mode selection of mode competition. By simulating the relative gain of each mode of the crystal waveguide core layer, the cut-off size of the core layer under different saturated light intensities in the cavity is calculated. A large-core size crystal waveguide was fabricated, and the output characteristics of the large-core size crystal waveguide active Q-switched pulse laser was experimentally studied. By using a 1.0% (atomic percent) Yb:YAG with core size of 320 μm×400 μm, 0.5% (atomic percent) Er:YAG with cladding size of 7 mm×30 mm and length of 77 mm single cladding rectangular crystal waveguide, and a BBO Electro optic Q-switch, the pulse energy of 1.29 mJ@10 kHz, the pulse width of 10 ns were obtained, and the M2 value of beam quality was 1.15×1.10. Experimental results show that the high peak power pulse output with near diffraction limit can be obtained by the actively Q-switched pulse laser with large core size diameter crystal waveguide.