Taking advantages of flexibility and efficiency, multi-robot system can be used for machining the complex large-scale aerospace structures. However, there are problems such as imbalanced task allocation and difficult to eliminate interference between robots. To solve the above problems, a multi-robot cooperative assembly task planning algorithm for large-scale aerospace structures is proposed in this paper. The first step is to adapt a rotating planning strategy for large-scale aerospace structures to reduce the number of processing station and shorten the processing time of multi-robot system. The second step is to develop a task auction algorithm based on market method to balance the workload between each robot and improve the work efficiency of multi-robot system. Finally, simulations are shown to illustrate the efficiency of the proposed algorithm.