The cementing mud flowmeter is an instrument that is used to measure mud flow rate in oilfield cementing engineering. It belongs to tangential turbine flowmeter. To explore the influence mechanism of fluid conditions on its metering characteristics, the mathematical models of the driving moment and the resistance moment of the impeller of the flow meter are firstly formulated. Based on this, the instrument coefficient K model is established. And the fluid viscosity is confirmed to be one of the influencing factors. Secondly, in actual cementing operation, the influence of viscosity on meter measurement characteristics is complicated. Therefore, the finite element analysis software is used to establish the 6DOF impeller passive rotating fluid simulation calculation model. The flow field characteristics and instrument coefficient characteristics under the conditions of various fluid viscosity 35, 45, 55, 65, 75 mPa·s are simulated and analyzed. And the influence law of viscosity change on the flow meter measurement characteristics is summarized. Finally, the average error is 1. 38% by comparing the actual cementing measurement data with the simulation data, which evaluates the effectiveness of the simulation model.