This paper presents a new method: Zbus algorithm combined with the forward and backward algorithm to deal with power flow calculation of the distribution network with ring-net. The topological structure of the network can be analyzed by decomposing the branch network into radial parts and ring-net parts through the forward path which is the concept of the signal flow graph, using forward and backward algorithm on radial parts, calculation speed can be greatly improved by adopting parallel calculation; Zbus algorithm is used on ring-net parts, then the voltage and power of the boundary nodes will be corrected with the two parts calculation result to keep the whole network calculation in consistency. Finally, a practical example analysis shows that the method can not only processes the ring network problems very well, but also improves the calculation speed.