The invention discloses a device and method for turning main equipment automatically through a nuclear island ring crane. A trolley of the device is provided with a laser range finder used for detecting the traveling distance of the trolley. A laser reflecting plate corresponding to the laser range finder is arranged at the end of a ring crane portal frame. A hoisting distance sensor used for detecting the hoisting distance of the upper end of the main equipment is arranged on a hoisting mechanism. The device further comprises a hoisting control device and a controller. The hoisting control device is used for controlling the hoisting speed of the hoisting mechanism. The controller is used for controlling the traveling speed of the trolley according to the hoisting speed, the hoisting distance, the traveling distance and the length data of the main equipment in the main equipment turning process, so that the direction of a hoisting rope is kept identical to that of the gravity of the main equipment in the main equipment turning process. The method is used for turning the main equipment automatically through the device. Compared with the prior art, the hoisting rope is prevented from inclining when the main equipment is hoisted, and safety risks are effectively reduced.