Factors such as high gain and spectral width of pump laser will significantly broaden the output parametric spectra of optical parametric oscillators (OPO). Etalons, gratings and other components are usually used to control the spectral width, however, loss will be imported, and thus the output threshold of lasers will increase and the conversion efficiency and the output power will decrease. A scheme for obtaining mid-infrared laser with narrow spectral width and high power is reported, in which the spectral width compression components are not used. The proposed scheme obtains narrow spectral width and high power respectively. A narrow-spectral-width pump source with a 1.064 μm NdYAG main oscillator power amplifier (OPA) structure is built. By adjusting the power and the diameter of the pump light spot in PPMgLN crystal of the optical parametric oscillator, we can control the gain intensity in the crystal and the spectral width of mid-infrared laser. In the optical parametric oscillator, when the pump power is about three times of the threshold, 0.7 W, 2.9 μm seed laser with spectral width less than 1.12 nm is obtained. The seed light is amplified through two optical parametric amplifiers, and 6.27 W, 2.9 μm laser output is finally obtained, when the light conversion efficiency is 15.7% and the spectral width remains unchanged.