The hybrid method using a discrete element method (DEM) and an incompressible smoothed particle hydrodynamics (ISPH) method was developed to clarify the slag formation process during submerged arc welding. As a result, the weld pool formation and the slag formation processes were simulated simultaneously using the hybrid method. Moreover, the edge of the slag obtained by the simulation was thinner and the center of it was thicker, which were same tendency of actual slag. The maximum thickness of the slag near the weld line was equivalent to about 2.5 mm. Although the slag thickness obtained from the computation was a little thinner than the experiment, it supported the validity of this hybrid model. Thereafter, the ensemble average processing was carried out to discuss about the velocity field of a weld pool. The averaged result showed that velocity field of a weld pool was synchronized periodically with the transportation of a molten metal droplet.