Abstract: The quenching cooling medium is an indispensable medium for the rapid cooling of parts from austenitizing temperature to the target one in the heat treatment process. The maximum cooling rate of the quenching cooling medium, the temperature range of usage and the characteristic temperature would be the main factors considered by the heat treatment process developers in practical applications. In this case, the selected quenching cooling medium would not be the best choice when the actual cooling status of the parts in the quenching cooling medium, the material of the parts, and the structure of the parts are ignored. 20 MnCr5 intermediate shaft of the automobile reducer was chosen as the study case in this research,temperature measurement system（thermocouple + temperature recorder）, metallographic microscope, microhardness tester,micrometer etc. were used to test and analyze the specimens to select the appropriate quenching oil to reduce the distortion of quenching. The research results show that the quench oil HR228H and KR218H meet the quench structure and hardness requirement of 20 MnCr5 reducer intermediate shaft by using the continuous cooling curve and isothermal transition curve, and then t he HR228H oil with the smallest maximum cooling speed is selected from the candidate quench oils, which is the most suitable quench oil model. This study provides a scientific, reasonable and quantifiable method for the selection of quenching cooling medium.