Successive expand method and successive backward method are two commonly used heuristic algorithms for transmission network planning. The advantage of the former is that it can give a transition plan from the existing grid to the target grid. For the same planning problem, under the same objective function and constraint conditions, it finds that the planning results of the two methods are not consistent. The successive expand method has a little bit more amount of calculations and has not obtained the optimal solution. By analyzing the calculation steps and the effectiveness index of the two algorithms through the 18-node planning example, it is found that there are defects in the successive expand method in terms of effectiveness index and virtual lines. Therefore, this paper proposes an improved successive expand method, which is verified by calculation example to obtain optimal planning results. The relative merits of the two methods are analyzed and summarized, which provides a basis for the selection of transmission network planning methods This work is supported by Beijing Municipal Science & Technology Commission Project“The Transformation of Scientific and Technological Achievements—Research on Flexible DC Power Network Bringing New Energy Access to AC Power Grid Dispatching Technology”(No. Z161100004816025) and Science and Technology Project of State Grid Corporation of China “Research on Scheduling Technology of ±500 kV Flexible DC Power Network”.