Abstract：Through field observations and structural analysis and in light of the principle of classifying the joints according to their strikes and dips into conjugate joint sets and determining their phases, the joint structure developed in the Jinchuan dam area of the Dadu River was analyzed and studied. The authors classified the NE-, NW-, ENE- and ENE-striking and SW-, NE-, ESE- and WNW-dipping into four sets of conjugate joints, belonging to the regional NW- and N-S-trending structures respectively. According to this conclusion, the authors deduce that the formation and development of the joints in the area were affected and constrained by the regional Indosinian-Yanshanian NW-trending structural stress field and Himalayan N-S-trending structural stress field. The strata in the area dip SW, and the joints of the first phase dip in the opposite direction of the dip of the bedding of the strata, while the joints of the second phase dip in almost the same direction as the dip of the bedding. The joints of the second phase play the principal role in controlling the structural movement. In the civil engineering construction, joints of the two phases combine to cut the rock masses. The former is prone to crush rocks, while the latter is prone to form landslides. The study of the two phases of joints has great significance for the study of the engineering stability in the dam area of the Jinchuan hydropower station.