The invention discloses a multifunctional combined square tube. The multifunctional combined square tube comprises a steel tube, wherein the cross section of the steel tube is a square structure; an inner tube is mounted in the steel tube; a protective layer is arranged in the inner tube; a reinforcing block is arranged on the inner wall edge of the steel tube; a T-shaped slot is formed in the reinforcing block; a T-shaped slide block is arranged on the outer wall edge of the inner tube; and a reinforcing layer is arranged between the steel tube and the inner tube. In this way, the multifunctional combined square tube disclosed by the invention is simple in structure; the inner tube can be replaced, so that the service life of the multifunctional combined square tube is greatly prolonged; and the inner tube can be replaced according to the characteristics of transported objects, so that the extended use of the multifunctional combined square tube is increased.