Abstract: A method used to describe the stress-strain curve is firstly put forward. In this method, the influence of microstructure evolution is considered. On this basis, a new simulation method of microstructure evolution during hot deformation is also raised. Some key factors, such as the minimum local microstuucture volume and the minimum deformation incremental step are determined too. The prediction of mechanical properties after cooling is discussed subsequently. Finally, the two pass upsetting process of Q235 low carbon steel is simulated, and the test results agree well with that of the simulation.