Various power flow algorithms based on the centralized or distributed modeling to obtain the global wide area power flow for multi-area interconnected systems are proposed up to now， which have their own merits and demerits. The advantages of the methods based on the centralized and distributed modeling are tried to be combined in this paper and a mosaic algorithm for distributed power flow characterized by centralization is proposed. Admittance matrixes， nodal injected power vectors and node type information vectors of all subsystems are stitched together to form the global power flow equations in this algorithm. After the global power flow is gained at one subsystem using the global power flow equations， the power flow data of the other subsystems is transmitted back. The mosaic algorithm can get the same calculation result as that of the integrative modeling without the need of coordinative iteration calculation among the subsystems. An example of IEEE 9-bus system shows that the mosaic algorithm possesses prominent advantages.