The toxic gas produced by the underground large-scale contained explosions have a great impact on the surrounding environment safety of the explosion project, so it is of great significance for the safety protection design to predict the leakage of the explosion gases to the environment. Based on the study of the one-dimensional spherically (axially) symmetric fluid dynamic model for seepage of the underground contained explosion gases, the qualitative and quantitative variables which impact the gas seepage, such as the explosion cavity, the gas pressure in the cavity and the rock mass permeability, among which the qualitative variables can describe the uncertainty of the geological mass, were analyzed. In accordance with the physical significance of variables and the quantification theory, the qualitative variables were quantified and such details as the principle for variable selection, significance assessment and recurrence test to establish a quantification prediction model, etc. were discussed. Based on 15 test samples on the hard rock conditions, we have established a quantification model for prediction of the contained explosion gases leakage at the specific site. The prediction accuracy of this model can meet the requirement of this project, and according to this model, it is known that such qualitative factors as groundwater status and rock properties have significant influence on the gas leakage. In this study, the prediction model is established based on physical analysis so that the quantitative prediction model could have its rationality and effectiveness well ensured, and the research methods and results in this paper could be promoted and applied in the similar engineering practices.