Abstract: Cryogenic Permanent Magnet Undulator(CPMU)is an important insert of the third generation synchrotron radiation source,which can effectively improve the beam quality of the illuminant.CPMU needs to work at liquid nitrogen temperature to perform its performance,so the supercooled liquid nitrogen circulation system(SLNCS)becomes an important peripheral equipment to support the CPMU.According to the requirements of CPMU,this paper designs the flow chart and working mode of the circulating system,designs the structure of the cold box dewar and the automatic pressure control device,analyses and calculates the resistance of the system,chooses the suitable liquid nitrogen pump based on it,designs and calculates the supercooled heat exchanger combined with the heat load required by CPMU.The off-line and on-line tests of the supercooled liquid nitrogen circulating cooling system show that the design meets the cooling requirements of CPMU.