The Sirri jacket offshore platform was designed and installed in Persian gulf of Iran in 1975 and has been in service since that time. The importance of offshore structure's stability and the failure of this kind of structure during an earthquake in different active zones revealed that earthquake loading has to be considered. In this paper, Magnetorheological (MR) fluid dampers have been proposed as a powerful tool to control seismic vibration of platform. A combination of four MR dampers and six friction pendulum isolators on the joints of cellar deck is applied for dynamic control of an existing steel jacket. In order to accurately evaluate the performance of Sirri jacket with MR dampers under Kobe seismic excitation, the platform numerically modeled in SACS software. The size of generated model with 198 degree-of-freedoms (DOFs) was dynamically reduced so as to be utilized in semi-active control algorithm. To this end, the stiffness, mass and damping matrices of the model has been reduced to the 25 DOFs model by programming in MATLAB software. The algorithm is used in this study for semi-active control of platform was H2/LQG. Comparison between dynamic response of the jacket with and without using semi-active controller showed a great difference in quantity of joints displacement and acceleration. As a consequence, cellar deck joints displacement and acceleration reduced about 50% in average. The results of using semi-active MR dampers showed that the earthquake-induced vibrations can be effectively suppressed by the isolation layer with MR dampers.