Abstract: Numerical simulation calculation on welding sequence of bottom plate of large-sized structure is conducted by using non-line finite element model,and the sparse and close finite model is used.The thermo-mechanics parameters of changing with temperature and the high efficient string heat source model of double elliptical are utilized to qualitatively calculate the deflection distortion of different welding sequence and different constraints.The results show that the deflection distortion is smaller for the first welded bottom plates than the first welded stiffened plates,is smaller for the successive welding than the simultaneous welding of bottom plates,and is smaller for using rigid plate constraint welding than non constraint welding.The distortion of the stiffened plate for the successive welding will occur,but the simultaneous welding won't do.The deflection distortion of welded structure relates to the neutral axis of its structure.