In order to determine the mutual support capacity between regional power grids with complementary characteristics in terms of new energy output and electrical load demand，fully leverage the mutual support advantages of cross-regional interconnected grid capacity，reduce allocation of system reserve capacity，and enhance system operation reliability，the complementary characteristics between new energy output and electrical load demand in interconnected regional power grids and the feasibility of the mutual support between regional power grids are analyzed from the perspective of source-load matching.An algorithm for quantifying the mutual support capacity of the residual power after source-load matching is proposed.To fully exploit mutual support potential while considering economic efficiency，based on the principle that all regional power grids are given priority to participating in internal electric energy interaction，a spatial-temporal mutual support capacity optimization evaluation model of regional grids is constructed，which incorporates constraints such as internal and external power exchange within the system and residual power mutual support capacity after source-load matching，so as to minimize the overall operational cost of interconnected regional power grids.The Cplex solver embedded in MATLAB is employed to perform case simulation analysis and obtain the solution，thereby validating the effectiveness of the proposed model.