With the high penetration of distributed generation and the gradual increase of ring network structure, distribution networks urgently need current differential protection to overcome the challenges faced by over-current protection. Taking the opportunity of 5G commercialization, this paper develops a distributed differential protection for distribution networks using 5G. In this protection system, a sliced network based on mobile edge computing is deployed as a data channel. This has the feature of ultra-low latency and ultra-high reliability. The fault instant-based self-synchronization method is adopted to realize the data synchronization required by differential protection. It has no need for additional timing devices. In a demonstration of 5G application for the smart grid, the developed relays are comprehensively tested under both single and crossing base station environments. The relays have been put into use in a 10 kV feeder for test run purposes. Both field test results and test run data indicate that the performance of the proposed differential relays meet the requirements of a distribution network in engineering application. This work is supported by the Science and Technology Project of the Headquarters of State Grid Corporation of China (No. 5100-202055018A-0-0-00) and the Science and Technology Project of Shandong Electric Power Company (No. 5206011900DG).