Abstract: Because of size deviation and bolt hole axial misalignment after casting, the upper and lower cylinder of the steam turbine can't be assembled directly, it must be repaired. In order to evaluate the feasibility of repair welding technology, this paper simulated the multi-layer and multi-passed welding process and predicted the deformation of main size in the lower cylinder wall. Calculation results show that the mid-surface smoothness was within 2 mm after partition jump welding, the axial alignment tolerance of steam sealing was less than 1 mm, and its open size expanded first and then shrunk from throat to outlet and the maximum deformation was 0.886 mm. Repair welding deformation forecast provides the theoretical basis and technical support for making and evaluating the correct repair welding technology.