Abstract：The effect of reaction pressure on reaction activity and selectivity of dimethyldichlorosilane (DDS) in direct synthesis process has been investigated experimentally. The experimental results show that the reaction rate of this catalytic reaction increases with the increase of reaction pressure, with an almost constant silicon conversion. However，the selectivity of DDS will decline when pressure rises. This is brought by that the main and secondary reactions obey different reaction orders with regard to pressure. So high pressure is inadvisable and a pressure of below 0.3 MPa is recommended for practical production. The relation between reaction selectivity and operation pressure is regressed to predict the selectivity under the pressure range from 0.1 to 0.5 MPa. Furthermore, it was found in experiments that the influence of pressure fluctuation on the reaction performance is reversible. This demonstrates that the variation of pressure only changes the species concentration of gaseous phase and does not affect the properties of the contacted mass. Thus the pressure fluctuation in short time is reversible.