The invention provides a 3D printing system for a climbing single-axial single-rail building and a method, for solving the problems of lower printing efficiency and complex system control of the present printing system with only one printing head. The 3D printing system comprises a printing device and an electric climbing device, wherein the electric climbing device is arranged on an outer vertical surface of a printed building; the electric climbing device can drive the printing device to move along a vertical direction; the printing device comprises a pair of parallel rail beams, a printing head beam, printing head rods and a printing head; the printing head beam is arranged between the parallel rail beams and is capable of moving along the parallel rail beams; one end of each printing head rod is fixedly connected with the printing head beam; the printing head is arranged at the other end of each printing head rod; the printing head rods are densely distributed on the whole printing head beam; the bidirectional movement in a horizontal printing plane is converted to unidirectional movement, so that the reliability of the 3D printing system is increased; and the printing on one layer can be finished after the printing head beam moves once, so that the efficiency is increased.