Abstract: The Joule-Thomson （JT） cryocooler precooled by Gifford-Mcmahon （GM） cryocooler combines the performance characteristics of regenerative cooling and throttling cooling in different operation temperature zones. The system has advantages of high cooling efficiency, large cooling capacity, light weight and high space utilization rate, and has wide application prospects in the fields of superconducting suspension, biomedicine and deep space exploration. This paper carried out theoretical analysis and experimental test research on GM-JT cryocooler system. Firstly, the components of the GM-JT cryocooler, the functions of the components, and the working principle of the system are introduced. Then, under specific assumptions, the influence of pre-cooling temperature and high pressure on the unit mass cooling capacity, circulation flow rate and required pre-cooling capacity was theoretically analyzed. and the determination of the target operating conditions was completed. Finally, based on the existing equipment in the laboratory, the core components of the cryocooler machine were selected, a low-temperature testing platform was built, and the verification test of the cooling performance was completed. The measured results show that the GM-JT refrigerator can achieve 5.85 W@4.44 K cooling capacity, input power consumption is 10.5 kW, and relative Carnot efficiency is 3.62%, which verifies the rationality of the theoretical process design. In addition, the recondensation system can achieve zero evaporation operation of liquid helium under the 5.50 W@4.45 K heat load, which meets the technical index requirements of the net cooling capacity output of the cryocooler system ≮5 W@4.5 K. This research will effectively improve the supply chain system of each link of the cryocooler machine, achieve independent control of the core technologies of the entire refrigeration system, break the Japanese technological blockade on large-capacity GM-JT commercial cryocooler machines, ensure the domestic leading position in the superconducting electric levitation technology field, further lay a solid foundation for the commercial operation of high-speed maglev trains, and promote the development of the low-temperature refrigeration machine technology industry.