New requirements on the reliability of the detector are put forward for the application of infrared detector in some special environment. In order to study the effect of hygrothermal environment on PbS detector performance, the PbS films had been prepared by CMD, surface morphology of the PbS films were showed, performance and experiment between dry and hygrothermal environment were specified. The influence of Rd, Vs, Vn and D* between dry and hygrothermal environment were analyzed. The Rd of PbS photoconductive detectors had been incresed in hygrothermal environment and decresed in dry environment. The varieties of Rd was reversible between dry and hygrothermal environment, for the resistance of the PbS particles R0 and ΔR that had produced from the absorption of H2O in hygrothermal environment were constituted an equivalent series resistance, and ΔR changed with PbS detector in dry and hygrothermal environment, to form a kind of similar to the adjustable state variable resistance. The trend of Vn variety between dry and hygrothermal environment is same to that of the Rd, and on the opposition that of the D*.