The invention discloses a DC deicing line single-phase earth fault positioning method. The method is characterized by judging whether three-phase current at the AC side and positive and negative current at the DC side of a rectifying device in a DC deicing system meet a protection action equation, and carrying out corresponding differential protection action, wherein the protection action equation can effectively reflect various faults in a protection region of the rectifying device, and has a fault bridge positioning function. With influence of unbalanced current of two measurement loop current transformers being taken into further consideration, a conventional brake function for valve short-circuit protection is changed into a two-broken-line-type brake function, thereby improving protection reliability.