The primary-secondary connecting rod comprises a transverse frame, a vertical frame and a primary connecting rod, the transverse frame and the vertical frame are in a right-angle splicing shape, a threaded groove is formed in the side wall of the transverse frame, the end, close to the vertical frame, of the threaded groove penetrates through the side wall of the vertical frame, the inner wall of the threaded groove is fixedly connected with a secondary connecting rod, and the secondary connecting rod is fixedly connected with the secondary connecting rod. A clamping groove matched with the mother connecting rod is formed in one end of the child connecting rod, the child connecting rod is located in the threaded groove, and the inner wall, away from the mother connecting rod, of the threaded groove is in threaded connection with a circular plate. The child connecting rods are connected with the mother connecting rods through the clamping grooves, then the child connecting rods are connected through the bolts, then the circular plates are in threaded connection with the inner walls of the threaded grooves, then the child connecting rods are completely fixed, the connecting positions of the transverse frames and the vertical frames are fixed in the vertical direction, a plurality of screws do not need to be used for fixing, operation is easy, and practicability is high. And cracks cannot be caused in the transverse frames and the vertical frames, so that the use quality is ensured.