A comprehensive modelling analysis for the carbonylation stage of low temperature two-step methanol synthesis in an integrated two-stage reactor was conducted based on chemical reaction equilibrium and phase equilibrium. Dynamic behavior of the reaction process, the composition of the liquid and the gas phases, the effects of total pressure, space velocity, temperature on CO conversion, and CO reaction rate were simulated. The main features of the reactor behavior can be correctly predicted. In general, the composition of every component predicted corresponds well with the experimental data. The result shows that a high space velocity favors CO reaction rates at the steady state.