Based on the analysis of the mathematical model in Newton-Raphson method about three-phase distribution circuit, load, transformers and regulator, this paper puts forward a three-phase unbalanced power flow parallel computing method to improve the performance of large-scale complex distribution network computing. Integrating with feeder number and node number of the distribution network, the task assignment problems are designed in the isomorphism multicore processors. And the load balance and parallel speedup are analyzed and presented. Plenty of simulations are carried on the IEEE 123, a real 1 000 nodes and dozens of feeders, which are composed of IEEE 13, IEEE 34, IEEE 37 and IEEE 123, in order to test the convergence and computational efficiency of proposed algorithm. The simulation results show that the algorithm can reduce the calculation period, make full use of the resources of multicore processors, and significantly improve the computational efficiency.