The response time of magnetorheological fluid ( MRF) is a critical parameter that relates to the real-time controllability of magnetorheological smart actuators. However, the dynamic response of MRF involves a complex coupling from the circuit to the mechanical system, which requires addressing the challenge of decoupling from multiple parameters such as current, magnetic field strength, and shear stress. This article proposes a phase difference method based on the biased sine excitation current. By applying the biased sine current to excite sinusoidal magnetic flux density, the response time of MRF is obtained from the phase difference between the torque and the magnetic field, without facing the complex decoupling encountered in step response. In addition, this article breaks through the shear rate limitation of the general commercial rheometers through the design of the key mechanical parameters. It achieves MRF response time measurements at shear rates up to 3 000 s -1 . Based on this, this article designs and fabricates an MRF response time measurement device, and analyzes errors caused by eccentric assembly and magnetic field probe positioning. It is found that the response time of various samples is 23 ~ 140 ms under 1 ~ 3 A amplitude currents. In addition, the response time is first accelerated and then prolonged with the increase of sample concentration, which is negatively correlated with the external magnetic field strength.