To calibrate the equation of state for the detonation products of the pressed HMX/FOX-7 composite explosive， a Φ25.4 mm probe cylinder test was conducted. The JWL equation of state parameters were calibrated based on the expansion displacement-time history of the cylinder wall. Additionally， underwater explosion tests of the HMX/FOX-7 composite explosive were performed to validate the effectiveness and applicability of the calibrated parameters. The results demonstrate that the coefficient of determination （R²） between the simulated and experimental displacement curves reaches 0.9999， indicating the high accuracy of the calibrated JWL parameters. Furthermore， numerical simulations of the underwater explosion tests were carried out using the calibrated equation of state. Compared with the experimental data at stand-off distances of 2.0 m and 2.4 m， the relative errors of the simulated peak overpressure are -2.34% and 4.37%， the relative errors of the impulse are 9.3% and 1.43%， and the relative errors of the specific shock wave energy are -1.1% and -6.4%， respectively. Moreover， for the first bubble pulsation period and the maximum bubble expansion radius， the relative deviations among the simulated values， experimental measurements， and empirical formula predictions are all within 10%. The strong agreement between the simulations and experiments across both the cylinder and underwater explosion tests confirms that the calibrated JWL parameters exhibit high precision and excellent applicability， providing a reliable foundation for further research on HMX/FOX-7 composite explosives in underwater applications.