In order to solve the problem that rotating machinery vibration is too large around the critical speed, a two-span rotors system bench was established to simulate the starting process. Without changing original supports of the rotors system, two self-designed magneto-rheological dampers were installed at each shaft. Different working conditions was set to study the influences in different mounting position and operating current of dampers. The results show that the damper can reduce the vibration of the shaft where the damper was installed near the critical speed vibrations. According to test results, an on-off control method was proposed to control the two-span rotors’ vibration. The results show that the on-off control method can reduce the vibration when the rotors went through the critical speed of first and second order without stopping the machine.