Abstract: A 2D map fusion algorithm based on covariance intersection (CI) factor graph is proposed for the problem that the covariance is unknown in the process of multi-robot pose estimation. Firstly, the transformation from the robot coordinate system to the global coordinate system is realized by the coordinate transformation matrix. Then, the local estimation information weight is obtained based on the principle of minimized non-linear performance. The local estimation information is merged by the algorithm to calculate the pose and cross-covariance of the fusion point. Finally, the covariance common formula is used to calculate the probability constraint (covariance) of the fusion nodes to the next-level variable nodes, thereby the factor graph fusion is completed. The experimental results show that the algorithm is feasible.