In order to accurately calculate the flow number of asphalt mixture under SPT permanent deformation test, the shortcomings of existing calculation methods were analyzed. A local fitting method based on Francken model was proposed, and the optimal interval coefficient was determined. The results show that the error of the direct experimental value method is large, while the moving average value method is obviously affected by the step size. The calculation results of flow number are affected by the starting and termination conditions of the strain fitting curve, and the effect of the termination condition is more significant. With the decrease of the interval coefficient K, the right side of the strain rate curve gradually moves up and the left side moves down, and the flow number gradually decreases. When K<15, the calculated results of strain rate curves and flow number of the two mixtures almost remain unchanged, and the interval coefficient K of the optimal local interval in this experiment is 15.