The feasibility of the rigid-lip hypothesis was studied in the flow field simulation of the reaction tank in the step-feed A/O process. Two models were built. One model applied the rigid-lip hypothesis was called as rigid-lid model and the other was called free surface model. The results showed that the velocity distribution of the rigid-lid model is basically consistent with that of the free-surface model. On the vertical monitor sections, the error of the mean velocity between the two models is less than 8%. The results showed that the much less expensive rigid-lid model is applicable to simulate the flow field of reaction tank.