Multi-satellites scheduling is an extremely complex problem of earth observation. Before the scheduling, the preparatory work known as preprocessing is very important. It can reduce the complexity of the problem effectively. In this paper, the general process of preprocessing is introduced. And two major parts are discussed deeply. One is task normalization, the other is complex task decomposition. Both are closely linked. Task normalization based on XML is proposed. An algorithm of task decomposition is developed and the detailed process is described. Finally, the testing results show that the preprocessing method is both feasible and efficient.