Abstract: The heat-input to various welding pass has an effect on the welding deformation of the typical symmetric units of membrane water-wall,in the process of multi-pass welding which adopted the combination of numerical simulation of by ANSYS with thermal strain theory and GMAW welding parameters in actual production.The results show that the unit's deformation is not sensitive to the unconformity of the heat-input for single welding pass in the flat welding.However,the total change of heat-input to the welding passes in the flat welding affects the unit's deformation more sensitively,vice versa.The unit's deformation is greatly influenced by the unconformity of the heat-input for welding pass in the overdhead welding.At the same time,there is some contribution range of the heat-input to welding pass in the overdhead welding,which reduces the unit's total deformation.The unit's total deformation is diminished by the reasonable rational heatinput of overhead welding in the contribution range.On the base,the method of controlling the unit's welding deformation and computation model of the heat-input to each welding pass are brought up along with the experimental verification.It can be concluded that there are some contributive range which causes the decreasing of the total deformation of the heat input for overhead welding pass.