The longitudinal crack is a general potential vulnerability in the crest of earth-rock dam with concrete cutoff wall.However，the formation and propagation of longitudinal cracks in the dam crest increases the safety risk of dam.The longitudinal cracks distribution of Wujing Reservoir was investigated in detail in this study.Based on this investigation and monitor in a full operating cycle of the reservoir，the propagation law of longitudinal cracks with the variation of reservoir water level is obtained.With the deformation gradient method，the results reveal the deformation of the cracks are similar to the deformation gradient.