The invention relates to an automatic detecting device for the defect detection of an offshore oil platform, comprising four mechanical arms. The top ends of the mechanical arms are mounted on a platform, the bottom ends of the mechanical arms are provided with image collecting apparatuses or other non-destructive detection apparatuses. The device can compute the rotating angles of the mechanical arms according to input position coordinates to be detected, so that the mechanical arms can reach the positions to be detected. A control system comprises a PC and a single chip microcomputer, wherein the PC is used for finishing the computerization; and the single chip microcomputer is used for controlling a stepping motor. The automatic detecting device can improve the accuracy and the efficiency of non-destructive detection of an offshore oil platform and the operation safety.