Abstract: This paper applies the cloud model theory to engineering geology classification of the surrounding rock mass of tunnel. It calculates the cloud model membership degrees via instituting multi-level quantitative system and assessment level standards of the surrounding rock mass. It then determines the engineering geology classification of the surrounding rock mass by combining qualitative assessment with quantitative classifying results. Through engineering practices, it is shown that the cloud model theory unifies the ambiguity and randomness associated with the conventional surrounding rock mass classification. Its results are compared with the results that are jointly determined by several geological engineers. This comparison shows that the results of rock mass classification are identical in 14 sections among the total 15 sections. The new method has high accuracy and can be reasonably practicable for the classification of the surrounding rock mass of tunnel.