Abstract: An onedimensional homogenous mathematics model was adopted to simulate the adiabatic radial flow fixed bed reactor for dehydrogenation of ethylbenzene to styrene under vacuum. Effects of reaction temperature, H2O/EB ratio and pressure on conversion and selectivity were studied. A strict demand of pressure drop about the reactor system was pointed out under vacuum, and furthermore variable operation conditions were advised. The accuracy and reliability of the mathematical model were demonstrated by comparing the simulation results with plant operation data.