The invention relates to a decision method for optimizing an objective net support structure of a medium-voltage distribution network based on a reliability program. The method comprises the following steps of: actually analyzing a current situation distribution network to obtain a current situation typical net support structure, and performing reliability quantitative evaluation on the current situation typical net support structure; determining an alternative objective net support structure meeting a reliability requirement by using a reliability-based distribution network planning method and a reliability assessment method by drawing lessons from advanced experiences of distribution network net support structure at home and abroad; providing a transition program and the optimal reconstruction time for reconstructing the current situation net support to an objective net support; and finally, performing investment estimation and power distribution reliability cost/benefit analysis on a reconstruction program to obtain the objective net support structure which satisfies a reliability goal and has the optimized economical efficiency. For distribution network planners, the problems in three aspects of reliability goal setting, reliability-based distribution network planning and objective net support structure can be solved by using the method. By using the method, under reasonable investments, the expectant reliability level is achieved, and wasted investments are avoided.