By introducing VOF model to simulate the cooling liquid film in small thrust liquid rocket engine, numerical method was performed to calculate and evaluate the effects of the expansion size on combustion efficiency and heat transfer in the chamber. Comparison shows the calculation is reasonable. It has shown that as the dimensionless expansion height H^*increases from 0.111 to 0.222, combustion efficiency and specific impulse increase by 0.35 % and 0.5s. However, the dimensionless expansion length L^*plays a less significant role in the process. As it increases from 1.389 to 1.944, combustion efficiency and specific impulse decrease by 0.11 % and 0.3s respectively. It is evident that the heat transfer characteristics of thrust chamber keep stable in the variation of H^*or L^*. This shows in expansion thrust chamber, the main reason for combustion efficiency loss should be that gaseous MMH adjacent side wall cannot mix with gaseous oxidant efficiently. 