As classic drilling fluid rheological properties test instrument, Fann 50SL HTHP (high temperature and high pressure) rheometer has played a great role in the research and development of high temperature drilling fluid system. But the fluid is prone to climb upward when the original Fann 50SL HTHP rheometer is used to test rheological properties of high viscosity fluid, which will result in inaccurate test data and poor reproducibility of experiment, and even polluted bearing lead to serious problems of bearing rust and driving motor burning. Through the analysis on the flow characteristics of the high viscosity fluid and the measuring principle of Fann 50SL, the reasons influencing the accuracy of test data are found out, on this basis, 2 kinds of prevent-climbing structures are designed and installed on Fann 50SL HTHP rheometer, the climbing upward is effectively solved.