This paper presents a method for suppressing parasitic oscillations in crystalline waveguides, resulting in actively Q-switched laser output with high peak power and high optical-to-optical efficiency. The structural optimization of the Yb∶YAG/Er∶YAG single-clad crystalline waveguide incorporates three key enhancements: first, a highly absorptive Ge infrared coating applied to the cladding surface absorbs amplified spontaneous emission (ASE) photons in the cladding, effectively blocking parasitic oscillation pathways; second, the implementation of a slight refractive index differential between core and cladding increases the critical angle for total internal reflection of ASE at the interface, thereby reducing the grazing angle and decreasing the number of ASE reflections to block the reflection path within the core; third, the incorporation of a 0.5° tilt angle at the crystalline waveguide end faces eliminates residual ASE end-face reflections. Experimental data demonstrates that with an absorbed pump power of 67 W, the actively Q-switched crystalline waveguide generates an average output power of 15 W, a pulse energy of 1.5 mJ@10 kHz, a pulse width of 10 ns, and a peak power of 150 kW, achieving an optical-to-optical efficiency of 22% and a dynamic-to-static ratio of 57.7%. The beam quality factors in the x and y directions are 1.15 and 1.10, respectively, which are better than those of the conventional crystalline waveguides. Additionally, the crystalline waveguide maintains consistent output power without saturation at high pump power, confirming the effectiveness of parasitic oscillation suppression. This methodology establishes a novel approach for designing Q-switched/mode-locked lasers utilizing crystalline waveguides, offering substantial applications in laser processing, remote sensing, and related fields.