Abstract: In major hydraulic engineering, concrete structures are usually in the environment of high water pressure, and the dam has more or less cracks. However, most cracking concrete dams are still able to operate safely, which gives rise to the problems of whether or not to allow the appearance of cracks in concrete structures, how wide cracks that will not cause damage, and what a reasonable index can be used to describe and judge the extent of concrete cracking. In this paper, equivalent plastic strain represents concrete cracking damage in a smeared crack numerical model. Through the numerical simulation of concrete uniaxial tensile and based on experimental data and related literatures, the relationship between the equivalent plastic strain and principal tensile strain and crack width was established, and the value of equivalent plastic strain corresponding to “harmful cracks” was obtained. The method and results are applied to the study on the cracking for a concrete gravity dam, which has a good practical engineering significance.