Gas pipeline leakage poses great threat to the environment pollution and system security. The gas leak mass rate is a key index in the assessment of risk level during pipeline leakage. In this study, compressed air flow in a high-pressure pipeline with leakage is considered, and the leak mass rate is investigated experimentally and numerically. Three-dimensional unsteady simulations, with the dynamic mesh technology, were applied to investigate the transient onset of the pressurized air leakage. The numerical model was validated by the experiments through the comparison of the leak mass quantity ratio, defined as the ratio of leak mass rate to inlet mass rate. The rise time of leak mass rate increasing from zero to the peak value was compared at different test conditions. Relations of the peak leak mass rate to the inflow mass rate, pipe operation pressure, and leak diameter were scrutinized by various numerical cases. Based on the parameterization results, a non-dimensional correlation was proposed to predict the compressed air leak mass rate through a small leak hole in high-pressure pipeline.