Viscosity of the fluid is an important physical and chemical property index of the liquid. Zahn viscosity cups are used to check the viscosity of different liquids quickly in engineering. Each manufacturer produces a different cup. And a specific specification or standard has been noted in every enterprise when using these cups. ASTM D4212 is currently regarded as general international standards. The equation of converting the time of flow in seconds to kinematic viscosity in Zahn viscosity cup specification from Boeing or other foreign aircraft manufacturers is different from the general international standards (ASTM D4212). Based on the same standard viscosity liquid, the differences between the two specifications were systematically compared, and the reasons for these differences were analyzed, the possible effects of these differences were also given.