On the basis of traditional rotation method,a new online measurement method of rotational viscosity was proposed,which used the theory of hydrodynamics to analyze the viscous torque of rotor,simulated and calculated the viscosity of oil by establishing the relationship between the current of sensor DC motor and the viscous torque of oil.An online sensor for measuring the viscosity of engine lubricating oil was designed.The response of the sensor to the viscosity of lubricating oil was tested by high and low temperature test,and the sensor was calibrated.The results show that the sensor has a fast response and high sensitivity to oil viscosity changes,and its structure is simple,stable and reliable.It also overcomes the shortcomings of traditional viscometers,such as long measuring time,harsh environment requirements,large size and high price.