Abstract: The experiment of vacuum tightness is an important method to test whether the leakage of condenser vacuum occurs because air leakage is one of the major factors that affect the vacuum of direct air cooling unit. The mathematical model of vacuum tightness for air cooled condenser power plant is established through the theoretical analysis of heat transfer and thermodynamic processes. The changing trend of the back pressure with the experimental time is obtained, which will provide the basis data for analyzing the changing rule of vacuum tightness. A case of 200MW air cooled power plant has been studied, and the simulated results are very close to the actual measured data. The standard of vacuum tightness test for the direct air cooling units under different capacities is also discussed. Different power units should establish appropriate standards according to their respective vacuum volumes and designed air leakage volumes.