OBJECTIVE To verify the accuracy of the “water content-conductivity fitting curve” model for determining the critical point of microemulsion O/W phase, and to further use this model to optimize the microemulsion of tanshinone extract. OBJECTIVE The “water content-conductivity fitting curve” model was used to determine the phase critical point of microemulsion into O/W type. According to the proportion of microemulsion oil phase, total surfactant and water phase, the pseudo ternary was drawn using Origin 2021 software. The phase diagram was used to screen out the proportions of the larger milk-forming areas; the average particle size and PDI of the microemulsion were used as the inspection indicators, and the star point design-response surface method was used to optimize the microemulsion; high-speed centrifugation and standing at 4 ℃ and 25 ℃ for 60 days, to examine the stability of the microemulsion. RESULTS The optimized microemulsion formulation was(18.11% medicated oil phase, 61.21% Tween 80, 20.68% 1,2-propanediol, the water critical point is 64.29%), K_m was 2.96, preparation temperature was 27.4 ℃, under different experimental conditions, the stability of the microemulsion was good. CONCLUSION The O/W phase critical point determined by the “water content-conductivity fitting curve” model has high accuracy, and the optimized microemulsion has good stability, which achieves the expected research goals.