Abstract: Water vapor transmission rate (WVTR) is a key technical indicator for evaluating the barrier performance of thin films. Currently, common detection methods suffer from low accuracy and limited measurement ranges. Based on gas molecule permeation theory, this study investigates the physical processes of adsorption, dissolution, permeation, diffusion, and desorption of water vapor through thin films under various temperature and humidity conditions in a vacuum environment. It analyzes the testing methods for barrier performance and the fundamental characteristics of relevant test systems, and proposes a design scheme for a high-accuracy WVTR test system centered on mass spectrometry analysis technology. A test system device was developed, which achieves micro-leak sampling via a needle valve and accurate measurement of the water vapor partial pressure penetrating the film using a quadrupole mass spectrometer. The water vapor transmission rate is then derived through theoretical conversion, thereby resolving the technical issue of low testing accuracy inherent in traditional methods. Furthermore, experimental studies were conducted on the performance characteristics and influencing factors of PET film WVTR under different temperature and humidity conditions, yielding characteristic curves that affect the film's barrier performance. The results indicate that increases in temperature and humidity raise the water vapor concentration in the gas chamber, enhance the adsorption rate and diffusion coefficient, intensify molecular thermal motion, and consequently lead to an increase in the water vapor transmission rate. This elucidates the physical mechanism by which temperature and humidity regulate the barrier performance of PET films, providing a theoretical basis and engineering design reference for optimizing the design of high-accuracy water vapor transmission rate test systems.