To solve the problem of large welding distortion which leads to the low assembly accuracy and thehigh cost physical test. Numerical simulation is used to study how the reinforcing controls the welding distortion bybuilding the finite element model based on the similarity theory of welding temperature field. Two types of reinforcing are designed and contrasted the ability to control welding distortion. Through the simulation results，the distortion rules in three directions of diaphragms shell are known. The three directions are axial direction，radial direction and circumference direction. The conclusion is that the ability to control welding distortion of type A is betterthan type B’s and the welding distortion of excircle is larger than incircle’s in three directions.