In order to measure the absolute spectral responsivity of long-wave infrared detectors with high accuracy, improve the long-wave infrared laser power stability and beam quality, a set of long-wave infrared laser source was developed. In this laser source, a feedback-control system, based on the electro-optics effect of CdTe crystal, was designed to improve the stability of long-wave infrared laser power and a long-wave infrared spatial filter, based on the propagation properties of laser beam, was designed to improve the long-wave infrared laser beam quality. The parameters of the spatial filter were also analyzed in this paper. Experimental result shows the laser power stability is elevated to above 0.1% and laser beam quality is improved obviously. The reliable laser source is provided to measure absolute spectral responsivity of long-wave infrared detectors with high-accuracy.