In this paper, an effort is made to utilize mixed-gases as refrigerant in the open cycle Joule-Thomson (J-T) cryocooler used for cooling infrared devices. The theoretical analysis of the thermodynamic performance of the mixed-refrigerant J- T cryocooler is made, including the refrigeration temperature, the cooling capacity, and the cool-down period. The principle of the component selection of the mixture is presented. A simple theoretical estimation model is developed to calculate the cool-down period of the J-T cryocooler. An experimental setup is also established to test the thermodynamic performance of a miniature open cycle J-T cryocooler. The refrigeration performances of the cryocooler using pure nitrogen and mixture are compared theoretically and experimentally. Both the theoretical and experimental results show that using appropriate gas mixture can improve the thermodynamic performance of the open cycle J-T cryocooler used for cooling infrared devices.