Abstract: In this paper, using the nonlinear finite element analysis, the stress-strain in the Hua Liang Ting Earth Dam is calculated to investigate the cause of longitudinal cracks in the watertight core. In order to calculate the tangent modulus Et and Poisson's ratio μt, the hyperbolic mathematic model which was developed by Duncan-Chang is used. In this calculation the mid-point incremential method and quadrilateral elements are adopted and the vertical displacement revision and the stress adjustment of broken elements are considered. The calculated vertical displacement is in agreement with field experimental data. The calculated results show that when the water pressure in the borehole of water tight core exceeds minimal principle stress plus tensile strength, the longitudinal cracks are initiated.