The invention relates to a folding container and a side plate locking device thereof. The folding container comprises an underframe, a top plate, two foldable side plates, a turnable front end plate and a turnable rear end plate. Each side plate consists of an upper side plate and a lower side plate between which a plurality of side plate locking devices are arranged and comprises a first throughhole, a second through hole, a locking bolt, a limiting part and a locking part, wherein the first through hole is formed on a lower boundary beam of the upper side plate; the second through hole is formed on an upper boundary beam of the lower side plate corresponding to the first through hole; the locking bolt is provided with a bolt rod and a bolt head positioned at the upper end of the bolt rod and the vertical projection of the outer contour of the locking bolt is at least positioned in the first through hole; the limiting part is provided with a limiting opening and is rotatably arranged on the lower boundary beam of the upper side plate through a rotary shaft; the locking part is arranged at the lower end of the bolt rod; the locking bolt is mounted in the second through hole and the first through hole when the side plates are in an assembly connection state; the bolt head is matched with the limiting opening to limit above the lower boundary beam of the upper side plate; the lower end of the bolt rod is matched with the locking part to lock below the upper boundary beam of the lower side plate; and the locking bolt is separated from the first through hole when the side plates are in a separating and folding state.