In order to measure the absolute spectral responsivity of mid-infrared detectors with high accuracy, the improvement of power stability and beam quality of mid-infrared laser was theoretically and experimentally demonstrated. The power stability of mid-infrared laser was improved to be better than 0.1% by using acousto-optic modulator and feedback-control electronics. Based on propogation properties of Gaussion beam, the parameters were calculated and experimental setup of spatial filter was constructed. Owing to the spatial filter, the beam quality of mid-infrared laser was improved. A stable and reliable laser source is provided for the measurement of absolute spectral responsivity of mid-infrared detectors.