Abstract: The toroidal magnet and poloidal magnet of HT-7U superconducting tokamak are cooled by forced flow supercritial helium of 3.8 K. The basic capacity of the helium refrigerator is 1 050 W/3.5 K+200 W/4.4 K+13 g/s·LHe. The flow sheet of the refrigerator and the thermodynamic analysis of the refrigeration cycle are presented. The optimization is conducted with the two independent variables-outlet temperatures of the two turbines in series. The optimum parameters of the cycle and their influence on the cycle efficiency are found through the optimization.