Abstract: It is necessary to determine the shape parameters of the failure surface and stress conditions of soil when the limit equilibrium method is adopted to calculate the bearing uplift capacity of a deep belled pile. Based on the published model test results, a preliminary discussion is given on the above problems with a unified failure mode for belled piles with short or long pile lengths. According to the concept of Mohr stress circle, if only the self-weight is considered, the normal stress of the failure surface is obviously less than the actual one around the enlarged base and leads to a larger influence height. Based on the model test results, the stress coefficient on the failure surface is back calculated, and it is found that the value of coefficient is in the range of 1.3 to 2.7, which is insensitive to the base angle. The effect of embedment depth on the stress coefficient is not considered, which needs further studies.