The invention discloses an isolation method for a single-phase earth fault, and belongs to the field of emergency protection devices. The method comprises the steps that the duration time of the single-phase earth fault is measured and recorded; if the duration time of the single-phase earth fault is shorter than the set time, single-phase earth is judged as instantaneous earth; if the duration time of the single-phase earth fault is longer than the set time, the single-phase earth is judged as permanent earth; the instantaneous earth and the permanent earth in the single-phase earth are differentiated; and a first treatment measure or a second treatment measure is taken respectively. The instantaneous earth and the permanent earth in the single-phase earth are differentiated, and the different treatment measures are taken, so that a permanent earth fault is isolated quickly and accurately, expansion of accidents due to power frequency overvoltage of a system for a long time is avoided, and an instantaneous earth fault can be compensated accurately to allow the instantaneous earth fault to disappear automatically; tripping operation is not required; and the safety and the reliability of the system are improved. The isolation method is widely applicable to the field of system protection of a transformer station.