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Hybrid J-T cryocooler is commonly employed in space detective missions requiring liquid helium temperature. In fact, nearly all the space applications of mechanical cryocoolers working at 4.5 K, having been launched or under development are hybrid J-T cryocoolers. For instance, mechanical cryocoolers used in Planck, JWST, SMILES, SPICA and so on, are all hybrid J-T cryocoolers. However, there is hardly any research on space 4.5 K hybrid J-T cryocooler in China. Key laboratory of space energy conversion technologies in Technical Institute of Physics and Chemistry (TIPC), Chinese Academy of Sciences (CAS) has developed a 4.5 K J-T cooler precooled by two-stage pulse tube cryocooler. Base on this cryocooler, the influence of supply pressure and diameter of the J-T orifice on the cooling capacity of the J-T cooler is analyzed and experimentally studied. First of all, the influence of supply pressure and diameter of the J-T orifice is theoretically calculated. Then, experimental research is carried out. For the purpose of shortening experimental time, a two-stage GM cooler instead of pulse tube cooler is used to precool the J-T cycle. Firstly, open loop experiments are conducted because it’s much easier to adjust the supply pressure precisely. The mass flow rate of the J-T cooler rises monotonously as diameter of the J-T orifice decreases. However, the cooling capacity of the J-T cooler goes through an up and down process because the J-T effect turns to be worse when diameter of the J-T orifice increases. That is to say, performance of the J-T cooler turns to be worse when the diameter of the J-T orifice is larger than 34 μm. Based on the research of this paper, 34 μm J-T orifice may be the best choice of the hybrid J-T cooler. Besides, both mass flow rate and cooling power increase as supply pressure increases when supply pressure is below 2.0 MPa. In addition, the no load temperature of the J-T cooler rises with increasing supply pressure which is attributed to the flow resistance of the counter-flow heat exchangers. Secondly, three-stage oil-free linear J-T compressors are used to drive the J-T cooler, and the suction pressure and supply pressure of the three-stage compression system are 0.12 and 2.0 MPa, respectively. Finally, cooling power of 81.5 mW @4.48 K is achieved when the total power consumption of the J-T compressors is 102.7 W. And the temperature fluctuation at the evaporator is lower than ±0.05 K because the cooling capacity of the J-T cooler is provided by the vaporization of liquid helium.