<p>With increasingly stringent requirements for the airtightness of high-speed train bodies, determining appropriate airtightness levels has become critically important. To calculate the airtightness of high-speed train bodies more accurately, based on one-dimensional isentropic flow theory, this study derives cabin pressure calculation models for both positive and negative pressure conditions during static airtightness tests of high-speed train bodies. Since the flow coefficient, which is closely related to the leakage characteristics of the carriage, is influenced by multiple factors including operating pressure conditions (positive/negative), leakage path cross-sectional shape, and size, a flow coefficient calibration method is proposed to achieve high-precision and efficient calibration of the flow coefficient for trains with varying leakage properties. This method generates a series of flow coefficient values for circular and square cross-sectional shapes under both positive and negative pressure conditions across various cross-sectional areas. Furthermore, functional relationships between flow coefficient and leakage path area under positive/negative pressure are established through curve fitting. Using these functional relationships and the cabin pressure calculation model, the pressure variation curves for a static airtightness test are simulated. Specifically, for circular cross-sectional shapes, the theoretical curves under positive and negative pressure conditions exhibited <i>R</i>² values of 0.9936 and 0.9931, respectively, when compared to experimental data, and for square cross-sectional shapes, the corresponding <i>R</i>² values are 0.9928 and 0.9932, validating the accuracy of the proposed theoretical model. The proposed theoretical model effectively evaluates the airtightness of high-speed train bodies with varying performance levels during static airtightness tests, providing a robust theoretical reference for optimizing high-speed train airtightness design.</p>