Phosphotungstic acid method is a classical method to measure the Cationic degree of zwitterionic surfactant, and the method can be also used to determine the content of cations or zwitterions of the drilling fluid materials. It is not easy to determine the organic cationic degree of drilling fluid materials containing organic cationic groups accurately due to the complex composition and the deeper colour of sample solution. In this paper, products was purified first by cation exchange resin of sodium type, then the phosphotungstic acid standard solution was used to determine the organic cation degrees quantitatively with congo red as indicator in weak acidic conditions. The experimental relative error was 0.59%. In the paper, the method, principle, operating procedure and method validation was introduced. The results showed that the organic cation content of the drilling fluid could be determined with phosphotungstic acid method, which is free of anion and inorganic cations. The core relative recovery experiment also showed that the measurement results fitted in well with the usability, and the results were reliable.