Abstract: Based on the numerical simulation, the second law of thermodynamics is applied to analyze the irreversible losses in a miniature pulse tube cryocooler (PTC) for space application. Simulation results show that the regenerator is the main part generating irreversible losses in the PTC. The performance of double inlet PTC is better than that of orifice PTC. From the view of the second law of thermodynamics, the reasons of this improvement are as follows. Firstly, the helium gas through the regenerator is reduced, and then the irreversible losses in the regenerator, the connecting tube (between the compressor and regenerator) and the cold heat exchanger are reduced. Thus the power consumed by the compressor is less. In the mean time, the heat transfer between the gas and matrix is performed better, and the net refrigeration power is increased consequently. Simulation results show that Carnot efficiency of the PTC increase from 14% to 16 9% when the cooler change from orifice type to double inlet type.