Abstract: The rock mass of Zaoshi dam foundation is quartz sandstone with high compressive strength and low deformation modulus, showing characteristics of hard, brittle and broken. Under undisturbed conditions, the rock mass strength is high, however once disturbed, the integrity of the rock mass decreases greatly, showing obvious excavation relaxation features. According to the evaluation system in the existing specifications, most of this type of rock mass does not meet the quality requirements of the rock mass used as the foundation of a dam. However, if the foundation rock mass is determined in strict accordance with the existing specifications, the excavation depth will be very deep. According to the experience and lessons of similar projects, we will face a dilemma of rock relaxation increasing with excavation process. Therefore, it is necessary to establish a set of reasonable evaluation methods for this kind of rock mass, so as to effectively use it as the dam foundation rock mass. It is proposed to comprehensively evaluate the basic quality of rock mass from the aspects of rock formation lithology, weathering characteristics, structural type, integrity, physical and mechanical properties, and water permeability, and to study the difference of rock mass engineering characteristics, mechanical properties and rock mass quality in the vertical direction. Based on these results, we propose an evaluation criterion for rock mass quality to determine a reasonable foundation elevation. By this criterion, the elevation of foundation surface would not be determined based on the integrity and acoustic wave velocity of rock mass, but determined by the weathering variation degree and mud filling condition of rock fissures. As a result, the foundation surface was raised by 10 m to 15 m compared with the existing standard method, and the excavation quantity and concrete filling quantity were reduced, the construction period was saved. The proposed method has been verified by engineering practice, and the achievements have certain engineering reference value.