In order to investigate the temperature field and the distribution of the volume fraction of mixed gas in methanol reforming reactor, a two-dimensional mathematical model was presented for describing the complex physical and chemical processes in the reformer of methanol-steam. The numeration of the steady field in the reactor was made with software STAR-CD by using TGrid mesh technology and laminar flow model which dispersed the equation by finite volume difference method and solved by using SIMPLE arithmetic. The simulation result can describe the temperature field and the distribution of volume fraction of mixed gas in methanol reforming reactor. The model reliability was verified experimentally on self-designed equipment. The model can predict the temperature and volume fraction of various compositions, because the experiments dates are very close to simulated ones. The effect of the tube diameter on the reactor was investigated by using the model. The result shows that with the increase of diameter, the temperature is increased, but there is nearly no change of compositions volume fraction.