Abstract: J-T valve is commonly used in the last cooling stage of a helium refrigerator.The parameters of J-T stage affect the refrigerating power of refrigerator or the liquid yield of liquefier.The dependence of refrigerating power or liquid yield are given on these parameters:the pressure before J-T valve,the warm end temperature and the efficiency of the last heat exchanger.The optimum working state of J-T stage is obtained in different refrigerating modes.