The high power 660 nm semiconductor laser is fabricated with non-absorbing window structure by Zn diffusion method. The photoluminescence wavelength of the active layer near the window region blue shifts by 61 nm with the selective Zn diffusion in the window region of the chips, which effectively reduces the optical absorption of the cavity facets. The stripe width is 150 μm and the cavity length is 1000 μm. The chips are sintered with AuSn solder by p-side down onto the AlN heat sink. The packaged device shows the highest output power of 4.2 W without catastrophic optical damage. The horizontal divergence angle of the semiconductor laser is 6° and the vertical divergence angle is 39°. The emission peak wavelength of the laser is 659 nm at room temperature and the current of 1.5 A. The 10 semiconductor lasers are aged at the current of 1.5 A in continuous-wave mode with the simple air-cooled heat dissipation condition, and no failure has ever occurred for 4000 h. Therefore, the 660 nm semiconductor laser with watt-level output power has the advantage of high reliability and low operating cost.