In power distribution system, the ratio of resistance and reactance is comparatively high, making the classic fast decoupled load flow method unsuitable for load flow calculation in distribution system anymore. Therefore, a kind of coordinate rotation transformation method is proposed. By combining this transformation method with the classic implicit Zbus Gauss method, the decoupled load flow method is successfully deduced for distribution system. Taking the IEEE 33-bus, IEEE 69-bus, and an actual 145-node distribution system as examples, this paper tests this decoupled load flow method in detail. The results demonstrate that the proposed decoupled method is of high efficiency and practicability since it does not influence the convergence property of classic implicit Zbus Gauss method.