To solve the distance protection measuring loop hidden failure problem, this paper proposes a diagnostic method based on adjacent lines topological relations. Aiming at simple network and network with branch lines, it analyzes measurement values constraint relations among different protection devices in the fault duration. Constraints discriminants and thresholds are deduced, by substituting measure impedance and composing the results, the purpose of positioning failure device can be achieved. According to the problem of protection measure impedance sync issue, it puts forward a processing method by using measure impedance on faults steady moment. PSCAD/EMTDC is used to simulate failure under different short-circuit phases, which proves device within hidden failure could be recognized by using measure impedance on faults steady moment.