Because of the large section size of the gravity dam, the volume of concrete expense is huge. How to optimize the dam shape to make it safaty and economy is a problem needed to be solved in design. However, when the gravity dam is working, the upstream heel area may appear cracks. With the enhancement of dam height, the probability of occurring cracks in the heel area is great. But the cracks in heel area dont mean the dam to be destroyed. The dam can work normally under the crack existence. In the paper, the form optimization of the gravity dam subject to crack constraint is studied based on this circumstance. A mathematical model used for the optimal design of dam crack constraint is established, in which a crack constraint expression is let in. By an illustration, a reasonable section is obtained and compared with original one. The optimal section is considered as ideal.