With extensionable evaluation model, the quality of rock mass of petroleum storage cavern sealed by groundwater is classified in this paper based on Extenics Matter- Element model. According to the influencing factors of rock mass quality classification, the classical field and segment field are determined. Meanwhile, by means of weight coefficients obtained by mean square deviation and the introduced correlation degree, an extensible evaluation model is made for the rock mass quality. This model is applied in the Huangdao cavern sealed by groundwater, and good prediction result has been obtained. By sampling analysis, it shows that even though the result differs somewhat from that of BQ method and RMR method, this model is more reasonable, and is capable of reflecting the grade of rock mass quality.