In order to reveal the inherent relation of geological data and reflect deeply the engineering-geological characteristic of the jointed rock mass. In this paper, The fractal characteristics of discontinuity in rock mass was analyzed, put forward a new method to evaluate rock mass quality (RMQ) by using the relations of fractal distribution and density of joint, rock mass designation (RQD), and the distribution of interval between discontinuity in rock mass. The research indicated that the fractal dimension D can avoid the some shortcomings of CSIR method in evaluating the quality of rock mass, and can be used as a quantitative index for evaluating the quality of rock mass.