The turbidimetric method is a kind of simple and convenient way of rapid quantitative detection of dextran, which has been applied to membrane performance evaluation and testing. This work was based on the reaction mechanism of dextran solution and ethanol, and proposed that the reaction process is an important factor that affects the stability of the turbidity value of the final solution system. By improving the experiment condition, the accuracy of the dextran solution concentration was raised, and the problem that working curve and repeatability cannot be set up in testing was solved. Satisfactory results were achieved for linear range (100～1000) mg/L, detection limit (0.007～0.093) mg/L, and standard recovery (80%～95%) of dextran (20 K/ 100 K/ 200 K).