The invention discloses a design method of the completely-assembling frame of a low-rise building. The method comprises the following steps: 1) providing a calculation model of different initial rigidity values of beam and column nodes; 2) according to existing load and the initial rigidity values, making a bending moment diagram of a beam; 3) according to the bending moment diagram, calculating the sizes of the corresponding beam and column; 4) selecting the cost-optimal sizes of the beam and the column; 5) calculating the amount and the position of connection bolts on the beam column nodes to cause the initial rigidity of the node to be equal to the set initial rigidity of the node; and 6) utilizing a semi-rigid calculation method to judge whether a plastic hinge occurrence position and an interlayer displacement angle meet requirements or not under earthquake load. The design method can quickly calculate the cost-optimal sizes of the beam and the column and reinforcements which meet bearing capacity and earthquake force requirements, and can greatly improve the design efficiency of a beam and column frame structure.