Abstract: The alpha energy-loss method is an effective and novel way to measure film thickness and thickness uniformity. But this kind of measurement should be carried out in a vacuum chamber. How to choose the appropriate vacuum degree is of great importance for improving the measurement precision and reducing the construction cost of the vacuum system. In this paper, we simulate the process of alpha particles with a power of 5.486 MeV going through air by using the software SRIM, calculate the energy loss of particles coming from Am-241 through different air distances between the source and the detector with different vacuum degrees and obtain the relationship between the energy loss and the vacuum degree. According to this relationship, combined with the stability and repeatability of the measurement of alpha spectrometer, we have found the method of determining the required vacuum degree in the chamber for measuring the film thickness with alpha particles. Based on the result, we get a conclusion that a vacuum degree less than 100 Pa will completely meet the requirement of sufficient accuracy when the distance between the source and the detector is 2-8 cm.