The structure of a rock mass directly controls its mechanical and hydraulic properties, and the distribution of rock block volumes can directly reflect the structural characteristics of the rock mass. Currently, many studies simplify the joint surfaces as planes and ignore the influence of roughness on the volume of rock blocks. In this study, the Hurst exponent (H) and root-mean-square height (R_q) were used to characterize joint roughness, and the influence of joint roughness on the quantity and volume distribution of rock blocks was investigated. The results show that: (1) H and R_q control the roughness of the joint, and the roughness increases with the increase of R_q and decreases with the increase of H; (2) the quantity of rock blocks is generally positively correlated with joint roughness, that is, it increases as the roughness increases; (3) the influence of R_q and H on the distribution of rock block volumes is mainly achieved by changing the proportion of small-volume blocks,  and the mean and median volumes of rock blocks decrease as the roughness increases; (4) when the joints are orthogonal and spaced closely, the variation in the quantity of rock blocks can be divided into three stages with respect to the roughness: a stable stage, an initial growth stage, and a rapid growth stage, where R_q and H jointly control the relative distribution range of each interval. Finally, based on the data collected by photogrammetry, we establish a joint model of the rock mass, and study the distribution of rock block volumes in a slope along a certain highway in Dalian, which verifies the correctness of the above conclusions.