A coupling structure, comprising: a shaft, which comprises a penetration-axle section, the penetration-axle section of the shaft having an end that is formed with a plurality of axial insertion troughs that are extended in an axial direction and arranged in an alternate manner, a tube, which comprises a penetration-axle hole formed in a center thereof and corresponding to the penetration-axle section of the shaft, an insertion engagement section being formed in the penetration-axle hole of the tube at an end thereof that is opposite to the shaft, wherein a plurality of radial insertion troughs, in the form of a recessed surface, are formed in an inner circumferential surface of the insertion engagement section; characterised in that the shaft further comprises a threaded locking hole being formed in a center of a surface of an end of the penetration-axle section of the shaft, and the coupling structure further comprises a connection assembly, which comprises a coupling block and a fastening member, the coupling block being disposed in the insertion engagement section of the penetration-axle hole of the tube, the coupling block having an outer circumference that comprises a plurality of radial insertion blocks corresponding to the radial insertion troughs of the tube, the coupling block having an end surface that comprises a plurality of axial insertion blocks corresponding to the axial insertion troughs of the shaft, the coupling block being formed, in a center thereof, with a through hole to receive the fastening member to penetrate therethrough and press thereon, an end of the fastening member being selectively screwed into and tightened in the threaded locking hole of the shaft; wherein the coupling block is fit between and pressed between the insertion engagement section of the tube and the penetration-axle section of the shaft to achieve insertion and engagement simultaneously in both an axial direction and a radial direction for positioning. a shaft, which comprises a penetration-axle section, the penetration-axle section of the shaft having an end that is formed with a plurality of axial insertion troughs that are extended in an axial direction and arranged in an alternate manner, a tube, which comprises a penetration-axle hole formed in a center thereof and corresponding to the penetration-axle section of the shaft, an insertion engagement section being formed in the penetration-axle hole of the tube at an end thereof that is opposite to the shaft, wherein a plurality of radial insertion troughs, in the form of a recessed surface, are formed in an inner circumferential surface of the insertion engagement section; characterised in that the shaft further comprises a threaded locking hole being formed in a center of a surface of an end of the penetration-axle section of the shaft, and the coupling structure further comprises a connection assembly, which comprises a coupling block and a fastening member, the coupling block being disposed in the insertion engagement section of the penetration-axle hole of the tube, the coupling block having an outer circumference that comprises a plurality of radial insertion blocks corresponding to the radial insertion troughs of the tube, the coupling block having an end surface that comprises a plurality of axial insertion blocks corresponding to the axial insertion troughs of the shaft, the coupling block being formed, in a center thereof, with a through hole to receive the fastening member to penetrate therethrough and press thereon, an end of the fastening member being selectively screwed into and tightened in the threaded locking hole of the shaft; wherein the coupling block is fit between and pressed between the insertion engagement section of the tube and the penetration-axle section of the shaft to achieve insertion and engagement simultaneously in both an axial direction and a radial direction for positioning.