Abstract: At present there are a lot of literatures referring to transmission network planning, but less about medium-voltage distribution network planning, especially for the network with cross-points of line corridors. Ant colony optimization (ACO) is a heuristic algorithm with positive feedback, distributed computation and greedy characteristic. It is very suitable to search optimal path in a graph but prone to fall into local optimum. This paper integrates ACO with spanning tree algorithm to solve medium-voltage distribution network planning with cross-points. The model of medium-voltage distribution network planning is established, which takes the minimum investment cost of expansion lines and minimum energy loss as objective functions, power flow balance as equality constraints, and maximum line current, upper and lower limit of node voltage as inequality constraints. Meanwhile，the network topology graph is radial and connected. The dynamic pheromone threshold of candidate paths and dynamic adjustment of path selection strategy are introduced in order to reduce the possibility of falling into local optimum in general ACO. According to the maximum iteration number, the current iteration number and the maximum pheromone value of all paths in the current iteration, the dynamic pheromone threshold of the candidate paths is given，which can ensure that the difference between the pheromones of the paths is as small as possible to increase the diversity of solutions at the early search stage, and the difference between the pheromones of the paths is as large as possible to speed up the algorithm convergence at the later search stage. The probability to be selected of one candidate path is decided by its length，investment and pheromone quantity. The shorter length, the smaller investment and the more pheromone can make the greater opportunity to be chosen. The dynamic adjustment of path selection strategy can help the search process tend to minimal objective function. Considering the radiation and connectivity constraints in medium-voltage distribution network, spanning tree algorithm with cross-points is proposed. Taking the power source and load points as the vertices of the graph, the spanning tree is produced by using depth-first search algorithm. Then the journey of each ant as a spanning tree is limited to a radial and connected grid, so infeasible solutions reduce greatly. After forming the path of one ant, do delete cross-point on path and its associated branches tentatively. If the deleted graph is disconnected, then retain the cross-point point and its associated branches, otherwise, remove the cross-point. To examine this method’s practical applicability, a practical 10 kV distribution line is used as an example for an empirical research. Simulation results demonstrate that the proposed method is feasible and effective for medium-voltage distribution network planning with cross-points. The dynamic pheromone threshold setting of candidate paths and dynamic adjustment of path selection strategy can improve the global search capability of ACO. The spanning tree algorithm can guarantee the radiation and connectivity of grid with cross-points, then the number of feasible solution increases greatly.