With the increasing uncertainty of source-load in the new power system, the allocation of reserve resources designed for short-term emergency power supply may be over-configured in long-term power system operation scenarios, resulting in a waste of grid planning resources, so proposing a two-stage reserve resource planning method that considers both long-term and short-term cycles is an important means to balance the supply security and operational economy of the power system. First, the key uncertainty factors that lead to redundant reserve resource planning are analyzed, including the uncertainties of new energy sources, loads, and line failures. A two-stage reserve planning model based on chance constraints for source-load-storage is then constructed. Subsequently, based on the decoupling characteristics of the two-stage planning problem, a single-stage solution method for the two-stage reserve resource planning model is proposed. The power supply guarantee constraint is embodied to further achieve the effective solution of the source-load-storage backup planning model. Finally, the effectiveness of the proposed method is verified in the modified IEEE-30 bus test system. The case study results show that, without significantly increasing the computation time, the proposed two-stage reserve resource planning method for source-load-storage can reduce the average power shortage by about 54.7%, lower the reserve planning cost by about 26.7%, and decrease the new energy curtailment rate by about 60.4%. Therefore, the proposed method can effectively enhance the system's power supply capability, improve the efficiency of new energy consumption, and promote the safe and economic operation of the power system.