Aiming at the “abnormal” phenomenon that the penetrating crack opening of a gravity arch dam crest along the arch direction increases when the temperature rises and decreases when the temperature falls,a three-dimensional finite element method was used to simulate the force transmission characteristics of longitudinal cracks on the dam crest.By introducing two-node element method, the opening degree variation of longitudinal cracks was simulated under two working conditions of temperature rise and temperature drop, and the physical causes of abnormal variation of cracks were analyzed based on the measured data of cracks.The analysis results show that the arc length of the upstream side arch ring is longer than that of the downstream side arch ring. When the temperature rises, the deformation of the upstream side arch is larger than that of the downstream side arch, which leads to the increase of the longitudinal crack opening at the riverbed.When the temperature drops, the deformation of the upstream side arch ring downstream is larger than that of the downstream side arch ring downstream, which leads to the phenomenon that the longitudinal crack opening of the dam crest at the riverbed is smaller or even in a closed state.