Abstract: Aiming to study the influence from the collective effect of the pressure between earthquake and water in the construction joint to the gravity under various working conditions, we took advantages of extended finite element method (XFEM) and interaction integral theory to build a mathematical model of dam crack fracture under the collective effect of water pressure in the construction joint and earthquake, with Koyna gravity dam as an example. During the process, the effects of different dam foundation and dam elastic modulus ratios, length of initial crack and joint surface water pressure distribution on dam crack propagation were studied as well. It is proved by the calculation results that the expanded length of crack gets weakened with the increase of the dam base to dam body elastic modulus ratio on the condition that the crack water pressure distributes evenly and initial length of crack is fixed; when the elastic modulus ratio of dam and dam base is certain and the crack water pressure distributes evenly, the crack propagation route gradually increases with the increase of the initial length of crack and gradually approaches the dam base surface; when the elastic modulus ratio of the dam foundation and the dam body as well as the initial crack length are fixed, as the water pressure of the construction joint coefficient increases, the crack expanding length gradually decreases, and the crack gradually expands to the rock foundation.