Abstract：Over-excavation is an inevitable fact in tunnel excavation. Based on the principles of the Mahalanobis Distance Discriminating Analysis (DDA), a distance discriminating analysis model is proposed in this paper to predict over-excavation of underground opening. According to engineering practice, the main factors affecting overbreaking in tunnels include the geological characteristics of rock mass and the geological constitution, which are considered in the forecast DDA model. Five parameters influencing the over-excavation of opening, including two groups of joints, one group of layer surface, extension and space between structure faces, are selected as geometric parameters. Twenty-one sets of engineering data in an underground opening are used as the training samples. Corresponding discriminating functions are obtained to classify the samples to be discriminated. The re-substitution method is used to verify the stability of the DDA model and the ratio of mistake-discrimination is equal to zero after the DDA model is trained. Some field survey data in a pump-storage hydroplant were used to test the discriminant ability of the DDA model. The results show that the prediction results are identical with the actual situation, with high predicted accuracy and low misdiscrimination rate. The DDA model can be used in practical engineering.