Qinglong Depot of Nanjing Metro Line 4 uses NRIET-CI-Ⅲcomputer interlocking system, of which interlock host adopts two out of three structure. This article describes the structure of the interlock host and the design of system interconnection architecture, interior communication, synchronization principle, safety work methods, etc. to meet the functional requirements, performance requirements, environmental requirements and RAMS requirements of the interlocking host system.