Liquefaction scheme based on the supersonic lifting-pressure principle of steam injector has good performance on outlet pressure and liquefaction speed. However, the physical proceed of equipment is very complex; many parameters haven't been contained in the theory equations. The paper established the theory equations which consider the heat and mass transfer between the steam and water. The effects made by the coolant water temperature, injection coefficient, convergent ratio, water undercooling and speed difference are analyzed. The results show that the water should be atomized as smaller as possible, the coolant temperature should be lower, and injection coefficient should be 6～8. If design is correct, the lifting-pressure ratio of the convergent-divergent type steam injector can reach three times than the original.