The utility model relates to an electric-arc detection device for a ring main unit. The electric-arc detection device comprises a housing, photoelectric detection elements, a signal processor and a junction box; the housing comprises a main housing, an inner housing and an outer housing; the inner housing has a hollow hemispheroidal structure, the inner housing is fixed to an inner side of the main housing, and a seal ring is provided between the inner housing and the main housing; a plurality of the photoelectric detection elements are uniformly disposed on the inner housing, and the photoelectric detection elements are photodiodes; the single processor is fixed to an outer side of the main housing, the outer housing and the main housing are fixedly connected, and the signal processor is wrapped in a cavity formed by the outer housing and the main housing; and the junction box is disposed on the outer housing, the photoelectric detection elements, and the signal processor and the junction box are electrically connected and are connected to a control system of the ring main unit through the junction box. Switch running states can be timely detected, controlled or prompted, maintenance processing can be timely performed, and the electric-arc detection device is safer and more reliable.