The authors proposed a switchback welding method to make stable back beads in the first layer weld during one-side multi-layer welding. In the proposed welding method, a welding torch is moved backwards and forwards like a switchback. The torch motion is important in the control of the weld pool. In this paper, the influence of the torch motion on the weld pool shape is investigated in order to find the optimum welding parameters. Firstly, fundamental welding experiments were carried out in thin plate one-side butt welding to determine the parameters of numerical simulation. Secondly, a simple numerical model for switchback welding was constructed and a heat conduction analysis was carried out, to investigate the effect of the travel speed and backward stroke distance of the torch on the weld pool. When the travel speed of the forward stroke was faster than that of the backward stroke, the weld pool length became shorter than a constant travel speed. On the other hand, if the backward stroke distance became longer than half of the forward stroke distance, continuous back beads could be obtained, regardless of such a disturbance as fluctuation of the arc length. The validity of the switchback welding method was verified by numerical simulation and fundamental experiments.