A guardrail combining structure is provided to reinforce the supporting force about external impact by quickly and simply combining a strut pipe with upper and lower pipes even without additional welding facility or electricity. A guardrail combining structure comprises an upper pipe(3) combined at the upper side of a strut pipe(5) by an upper pipe fixing member(10); a lower pipe(4) combined at the lower side of the strut pipe by a lower pipe fixing member(20); a casting decoration body(1) having various patterns in a square frame(1a) and combining protrusions(2,2a,2b) in the square frame to weld the upper combining protrusion on the upper pipe, the lower combining protrusion on the lower pipe, and the lateral combining protrusion on an auxiliary pipe(8); the strut pipe formed with a fixing groove(7) at the upper side to be fitted to the outer diameter of the upper pipe and a combining hole(5a) bored at the lower side; the upper pipe fixing member inserted into the inner side of the upper side of the strut pipe to fix both ends of the upper pipe inserted into the fixing groove; a reinforcing member(6) longitudinally inserted/attached to the strut pipe; the lower pipe fixing member inserted into the lower side of the strut pipe to fix the lower pipe; and a cap(17) put on the fixing groove. The decoration body is made of aluminum casting material, and the combining protrusion is made of a stainless material and formed with a fixing unit in the body of the patterns to be integrated with the aluminum casting body.