Abstract: The existing models of distribution systems security region（DSSR） do not take into account the network power loss and voltage drop constraints accurately, resulting in the error between models and real consequence of security analysis. This paper modeled the DSSR based on AC power flow considering network power loss and voltage drop constraints. The partial boundary of the model can be obtained by the approximation method. The proposed model and a conventional DC flow based linear model of DSSR were compared through a case grid, and the error between the two models were described, demonstrating that the proposed model in this paper is more accurate. The suitable range of application of the proposed model was determined by analyzing the relationships between the error and network power loss and voltage drop constraints, which provides a reference to the implementation of DSSR.