The extensive penetration of open-phase running distributed generators such as photovoltaic generators to distribution systems may lead to a serious three-phase unbalanced condition, which is an intrinsic characteristic of distribution systems. The effect of the traditional evaluation of the power supply capability for distribution network is poor without considering the unbalanced problem. In order to accurately analyze the influence of unbalanced three-phase characteristics on the power supply capability for distribution network, the evaluation model of power supply capability for unbalanced three-phase distribution network with distributed generators is proposed, which takes the maximum load parameter of the distribution network as the objective function, considers the state variables such as branch thermal constraints and nodal voltage, and the control variables such as active and reactive power of the distribution power supply. The phase with the most serious voltage sag is selected as the continuous parameter to ensure the continuation power flow algorithm of prediction and correction is accurate. Finally, the simulation result of the modified IEEE 33-bus three-phase test system shows that the proposed model and method are effective.