With a large amount of distributed generation connected to the medium and low voltage distribution network, three-phase power flow calculation is the basic means to analyze their influence. The existing research on three-phase power flow for a medium-voltage ungrounded distribution network mainly adopts the method of manually setting the reference neutral point or zero-sequence voltage. This makes it difficult to simulate and analyze the neutral point or zero-sequence voltage offset, and the zero-sequence power flow and the imbalance of phase to ground voltage. Thus this paper proposes a new three-phase power flow algorithm for a medium-voltage ungrounded distribution network. First, the particularity of three-phase power flow for this network in terms of node variables and admittance matrix is analyzed. Second, based on a power balance equation and the Newton-Raphson method, a three-phase power flow algorithm for an ungrounded distribution network with constrained zero-sequence current is designed by using admittance matrix reversible processing and matrix replacement technology. Finally, the power flow for the ungrounded distribution network is simulated based on the 6-bus system and the modified IEEE123 test case. The accuracy of the proposed method in different scenarios and its applicability and convergence in large power grids are verified by PSCAD, and a practical application is carried out in local power grids. The example analysis shows that the proposed algorithm gives accurate calculation results and good engineering practicability.