Shifting of the optical mode from symmetric distribution to asymmetric distribution by separate confinement asymmetric waveguide structure reduces optical absorption loss by carriers, and permits higher doping of the p side to reduce resistance. Based on the theoretical analysis for the optical field distribution characteristics of GaAsP/GaInP tensile-strained single quantum well (SQW) lasers with asymmetric waveguide, the thickness of waveguide layers was optimized, and a high-power semiconductor laser with Al-free active region was fabricated and studied experimentally. For a 900 μm cavity length device, the typical threshold current density is 400 A/cm2, and a low internal loss around 1.0 cm-1 is achieved. Under continuous wave (CW) operation condition, 150 μm aperture devices achieve a CW output power of 6 W, the maximum slope efficiency of the prepared devices is 1.25 W/A, and the lasing wavelength is 807.5 nm. Horizontal and vertical far-field divergence angles are 3.0° and 34.8°, respectively. The characteristic temperature of the laser in the range of 20～70 ℃ is estimated to be about 133 K. Separate confinement asymmetric waveguide structure is proven to be an impactful method for reducing optical loss and improving the characteristics of high power diode lasers.