In this paper, the physical model for flow field of exhaust pipe has been established on the condition of a liquid-gas exhaust pipe from cars using ethanol gasoline. The numerical simulation results for internal flow field of car exhaust pipe are compared with the experimental data. It is showed that the outside free vortex tangential velocity and axial velocity of single-phase flow field have been simulated better. It is indicated in the simulation of separation process of steam-water two-phase: The relative error, comparing the relationship between flow and separation efficiency with measured of that is less than 7%, and the same to 15%, comparing the relationship between flow and separation ratio with measured of that. Thereby, it is confirmed correctly for the mathematical model founded for numerical calculation and the algorithm selected.