The real time digital simulation（RTDS） is one of the most important means and state-of-the-art technologies used to investigate and analyze power systems. Block division and Rack assignment of large-scale power grids are the major problems in the construction of a RTDS model. Rational block division can improve the real-time simulation efficiency. Subgraph isomorphism is applied in the block division and Rack assignment in the construction of the China Southern Power Grid with RTDS system. This problem is converted into one of finding an isomorphic subgraph of the block connection graph in the Rack connection graph. Optimal allocation decision is achieved by searching the feasible allocation decision of Racks with the subgraph isomorphism algorithm. Calculation results show that the Rack's allocation decision based on the subgraph isomorphism algorithm is feasible.