Hydrogenated amorphous silicon (a-Si:H) thin films have been prepared by direct current (DC) magnetron sputtering. The effects of the hydrogen flow rate, sputtering power influencing on deposition rate and the optical characteristics of a-Si:H thin films have been investigated. The hydrogen content (CH) of the films is calculated by Fourier transform infrared (FTIR) spectroscopy method, and the maximum CH is obtained at 11% (atom percent). The refractive index (n) and extinction coefficient (k) are measured by spectroscopic ellipsometer. It is found that the n and k of the prepared film decrease with the increase of CH. The optimizing parameters are applied to the preparation of high reflection mirror of diode lasers, and the n and k at 808 nm wavelength are 3.2 and 8×10-3 respectively with a satisfactory laser output characteristics.