To solve the cable line distribution network protection impact of active distribution network bi-directional current, this paper analyzes the protection problem of the DER access and puts forward the scheme adapt to active distribution network differential protection. The scheme disposes centralized line differential protection and decentralized bus differential protection locally, which is suitable for cable line distribution automation. DTU adopts built-in Ethernet port technology, uses dedicated fiber channel hand in hand to connect. Based on fiber channel to realize synchronous sampling, DTU transfers current-in line and current-out line to centralized differential protection device, centralized differential protection device realizes distribution network line protection, DTU realizes ring main unit bus protection, which can well solve those problems of DG access. Corresponding DTU and centralized protection device are designed and used in the practical cable line ADN system, which has good application prospect.