The longitudinal crack problem at dam crest is a common engineering problem in earth-rock dams, and its existence and development increase the safety risk of engineering operation. Regarding the cracks on the top of an earth-rock dam reinforced with a concrete cut-off wall, the depth of crack development was detected using pit exploration, and the elevation of the dam top pavement was measured. Finally, the causes and development process of the cracks were simulated and analysed based on finite element method. The results show that the depth of longitudinal cracks on the dam crest is relatively shallow, which has little impact on the overall structural safety of the dam. However, excessive settlement may cause damage to the water stop between the bottom plate of the wave wall and the head wall of the concrete cut-off wall, affecting the integrity of the dam cut-off system. At the same time, due to the jacking effect of the concrete cut-off wall, the pavement concrete is under tension. When the tensile stress exceeds the yield stress of the concrete, cracking occurs.