Lubricating oil in machine tools, hydraulic devices plays the role of lubrication, cleaning, etc. With the operation of machine tools and hydraulic devices, the quality of lubricating oil decreases, and the viscosity of lubricating oil also decreases. In order to avoid machine failure, lubricating oil needs to be replaced before reaching the oil change index. For the viscosity sensor designed based on rotation method, the rotor is its core component, and its stress condition directly determines the final detection effect of the sensor. Based on the basic knowledge of fluid mechanics, combined with the Fluent simulation experiment, the viscous torque of the rotor was analyzed. And combined with the working parameters of the motor, the force of the rotor at different temperatures and viscosity was obtained, as well as the current through the motor at this time, so as to obtain the optimal rotor structure.