Abstract:  Objective To improve the accuracy of rapid assessment of discharge rate, drainage time, and liquid-level status for side-wall small-hole leakage from cylindrical storage tanks, a rapid assessment method for key leakage parameters is proposed for early-stage accident evaluation. Methods A laboratory-scale atmospheric cylindrical tank leakage platform is established, and variable-head leakage experiments are systematically carried out under orifice diameters of 3–10 mm to obtain the discharge rate, drainage time, and jet range. Based on the experimental data, the dynamic characteristics of the discharge coefficient with respect to hole diameter and Reynolds number are analyzed, and a corrected discharge-coefficient model is developed and incorporated into the drainage-process calculation. In addition, a jet-range prediction model is established by taking the hole diameter, liquid level, and effective head as input variables. Combined with observable jet-range information, a liquid-level inversion model is further constructed to estimate the internal tank state from external leakage features. Results The study shows that, within the tested diameter range of 3–10 mm, the discharge coefficient is not constant, and its value is jointly affected by the orifice geometry and Reynolds number. The average discharge coefficient for small orifices (3–4 mm) is approximately 0.88, which is 31%–45% higher than the classical sharp-edged orifice values (approximately 0.61–0.65). The corrected discharge coefficient reduces the drainage-time prediction error from 24% to below 5%. The jet-range prediction model achieves an R² of 0.99, and the mean absolute percentage error for liquid-level inversion is 1.93%. Conclusion A rapid assessment framework for side-wall small-hole leakage from cylindrical tanks is constructed by integrating discharge-coefficient correction, jet-range prediction, and liquid-level inversion, providing technical reference for rapid assessment at the early stage of leakage accidents. (17 Figures, 8 Tables, 35 References)