To reveal the gas seepage characteristics and air tightness evolution of underground gas storage cavern for compressed air energy storage under long-term charge-discharge cycling, a case study is conducted on the Chongzuo Gas Storage test cavern in Guangxi, China. Based on the coupling theory of thermal-hydro-mechanical (THM) processes and the thermodynamic theory of compressed air, the COMSOL Multiphysics platform is used to establish a fully coupled THM numerical model that reflects the actual dynamic operation of underground gas storage caverns. The model simultaneously considers dynamic variations in the temperature and pressure of compressed air during operation, high-pressure air leakage, the interactive relationship between gas seepage and porous medium deformation, and the resulting dynamic evolution of medium permeability. The results obtained are as follows: (1) Under long-term operation, the air tightness of the gas storage cavern is at risk of deterioration. The daily leakage mass of compressed air continuously rises with operating cycles, accompanied by a simultaneous decrease in the mass of compressed air inside the cavern. Under the designated working condition, the daily gas leakage rate increases from 0.25% to 1.36% in the later stage, exceeding the safety threshold of 1%. (2) The seepage field evolves continuously under long-term operation: the seepage range, pressure, and velocity of the compressed air progressively change with the charge-discharge cycles, exhibiting remarkable spatiotemporal heterogeneity and diffusivity. (3) Multifield coupling accurately reflects the leakage path and thermal response of gas in actual operation. When considering seepage, the thermal influence zone in the surrounding rock expands considerably and the local temperature is higher than that when seepage is not considered.