This work aimed to study the detection method for on-line leakage detection system of non-PVC soft bag, and to propose the judgment criterion for reasonable detection of soft bag leakage. Theoretical analysis was conducted for the pressure change of the samples under external pressure, and an experimental device for leakage detection of non-PVA composite film soft bag was used to test 300 samples of the same batch and specification. Then normal fitting of the experimental data of the intact samples and leaking samples was conducted, and the pressure drop data distribution of the qualified samples was analyzed using mathematical statistic method. The result showed that the pressure drop of intact samples was less than that of leaking samples under the same pressure. The criterion for leakage detection was: the tested soft bag was qualified if its decreased pressure was between 11.31 N and 34.68 N during the dwell time, otherwise, the soft bag was leaking.