A Pr Fe B- based cryogenic permanent magnet undulator prototype was under development for High Energy Photon Source- Test Facility. Design working temprature of magnets was below 85 k while the maxmium heat load of CPMU was about700 W@ 80 K. The design flow rate was 12 L / min. Sub- cooled liquid nitrogen was divided into two parts,flowing through cooling channels on each girder respectively. According to the influence of circulation flow rate on the liquid nitrogen temperature through CPMU,heat load of pum...