Strong earthquake has adverse impact on the petrochemical steel structures of refinery and lead to large economic losses and serious secondary disasters. We studied a reformer reactor of a large refinery, built the finite element model and set viscous dampers to reduce the seismic risk of petrochemical structures based on their characteristics. Then we analyzed the influence factors, including location, quantity and the damping parameters of the viscous dampers. The results show that the inter-story force and inter-story displacement angle of the floor where the viscous damper was setted decrease significantly, and the damping effect with setting viscous damper in the bottom is greater than that in the middle and upper part. Besides, the inter-story force and inter-story displacement angle decrease when the number of viscous damper increases, but the decrease amplitude reduces gradually. In addition, the influence of damping coefficient on the damping effect of the petrochemical steel structure is greater than that of the damping index. So the damping coefficient should be given priority to consider in the structure design for better seismic performance.