The response time of a rotor system on a magnetorheological(MR) fluid squeeze film damper was measured experimentally. The effects of rotating speed, step voltage and rotor imbalance on the response time were dealt with. It is shown that the MR fluid damper rotor system is a second-order dynamic system. The rapid response time defined by the time taken for the vibration amplitude to rise (fall) from 10 % (or 90%) to 90 % (or 10%) of the vibration amplitude difference between the initial and final states is 0.05~0.7s in applying a step voltage and 0.01~1.225s in dropping a step voltage, respectively. It is impossible for the response time to be within a few milliseconds. The response time is determined not only by magnetic field strength, MR fluid specification, rotational speed, but operation mode of the power supply. There is a zero initial delay time at either applying or dropping the step voltage, which is caused by the magnetizing or de-magnetizing process.