The size of natural gas ejector under a certain working condition was designed by using the method proposed by the UUSR Institute of Thermal Engineering. The simulations covering 70 groups of ejector models with different structures under different working conditions were conducted adopting RNG k-ε eddy viscosity model and the results were verified by experimental data, and the comparison between the exergy calculated by experiment data and the entropy generation by simulation indicated that the entropy generation analysis method was reliable. The value of entropy generation involving with viscosity, turbulent dissipation, heat transfer with finite temperature difference, and the laminar boundary layer near the wall in ejector was calculated respectively. It was found that the entropy generation due to turbulent dissipation accounts for about 97%, indicating that the friction causing by the turbulent fluctuation was the major part of energy loss. Turbulent entropy generation in ejector was closely related to its diamond shock, oblique shock and shear diffusion between primary and entrained fluid. The axial distribution and peak value of turbulent entropy generation were positively correlated with the position and intensity of the diamond shock in mixing chamber and oblique shock in diffusion chamber, while the radial distribution of turbulent entropy generation gradually transited from the shear boundary layer locating in the middle of jet core and the secondary fluid to the entire cross section. In addition, the performance of the ejector became worse with increasing turbulent entropy generation due to the oblique shock in the expansion chamber. For example, the entrainment ratio would decrease with increasing turbulent entropy generation due to the oblique shock in the expansion chamber caused by excessive expansion ratio. And the energy would be wasted due to the oblique shock in the expansion chamber in a lower compression ratio, although the entrainment ratio remained stable in this condition.