The invention provides a detecting device for surface structure defects of a construction. The detecting device comprises a loading platform, a hoisting mechanism, a guide rail, a mainframe loading hanging plate and a measuring mainframe, wherein the loading platform is arranged on a parapet wall of a to-be-detected construction and is provided with a walking device; the base of the hoisting mechanism is fixedly connected with the top of the loading platform; a main bracket is arranged on the base; a cantilever arm is arranged at the top of the main bracket; a guide rail bracket and a guiding pulley are arranged at the external end of the cantilever arm; a dragging device is hung at the inner end of the cantilever arm; the upper end of a lifting rope is connected with the dragging device while the middle part is resisted against the guiding pulley; the upper end of the guide rail is fixedly connected with the guide rail bracket; a counterweight device is arranged at the lower end of the guide rail; the two ends of the mainframe loading hanging plate respectively sleeve the guide rail while the tops are connected with the lower end of the lifting rope; the measuring mainframe is fixedly connected with the base of the mainframe loading hanging plate. The detecting device provided by the invention can be used for closely and stably detecting the outer surface of the to-be-detected construction and ensuring the accuracy of measurement.