Abstract: In order to solve the problem of pilot contamination in channel estimation of cell-free massive multiple-input multiple-output (MIMO) systems, this paper proposes a scheme of weighted graph based superimposed pilot assignment. Firstly, the influence of pilot contamination on superimposed pilot channel estimation is analyzed, and a new metric is introduced to represent the potential pilot contamination level among users. Secondly, the user-weighted interference graph is constructed according to the large-scale fading coefficient between the access points and the users, and the throughput optimization problem is transformed into the capacitated max k-cut problem. Finally, vertex swapping local search algorithm is adopted to realize the suboptimal allocation of superimposed pilot. Simulation results show that this scheme can effectively reduce pilot contamination, improve the estimation error of channel based on superimposed pilot and system throughput.