The pulse tube refrigerator has no moving part at the cold end, so it has advantages over other types of refrigerators in terms of reliability, cost, lifetime. Great progress has been made on the development of the pulse tube refrigerators .Due to the certain application of pulse tube refrigerator of 20K, theoretical and experiment study was done to investigate the oscillating flow and heat transfer in pulse tube refrigerators.1. The experimental system of single stage GM type axial pulse tube refrigerator has been established, the cold end temperature reached 18.1K with 6kW input power, which is the lowest no-load temperature for GM type axial pulse tube refrigerator, and cooling power 12 W@ 30K is obtained.2. Experimental work were carried to investigate the function of the phase shifter- double inlet to gain temperature below 20K. The two parallel-placed valves with opposite flow direction instead of traditional single valve configuration as double-inlet are introduced, which successfully control the DC flow.3. The numerical model that considers thermodynamic and hydrodynamic for the oscillating gas flow and the regenerator matrix in the pulse tube refrigerator has been developed. The model can be used to simulate low frequency pulse tube refrigerator and high frequency pulse tube refrigerator, and numerical results agree with those of experiment.4. In order to decrease the no-load cold end temperature to below 20K, multi-stage high frequency pulse tube refrigerators are popular. Through a serial of simulation, the geometry of the second stage regenerator is optimized.5. The effect of lead spheres are discussed to improve the performance of regenerator.