The invention relates to a welding deformation control aid of a large-size stainless steel cylinder, comprising an inner arc flat panel, an outer arc flat panel and jack screws, wherein the inner arc flat panel and the outer arc flat panel are flat panels one side of which is an arc side, and the arc radiuses of the arc sides are the same as that of the cylinder; the jack screws are fixedly connected to both sides of points of the plat panels opposite to a longitudinal welding line of the cylinder; and the top ends of the jack screws face towards the arc sides of the plat panels. When in use, the assembly datum lines of the inner circumference and the outer circumference of the cylinder are drawn out on a welding platform, and the arc segment of the cylinder is hung on the platform and takes the places according to the datum lines; the lengths and the angles of a non-line length segment at both sides of the longitudinal line under a free state are measured, and the aid is selected according to the non-line lengths and the angles; the aid is spot welded on the cylinder, and the longitudinal line of the cylinder is alternately welded; and the welding order and the jack screws on the aid are adjusted according to the deformation condition of the longitudinal line. The invention has the advantages that the working procedures of mechanical shape correction and the hot shape correction after welding of the cylinder are avoided; the deformation in a welding process can be directly observed and regulated by taking technical measures; and the aid has simple structure and use method.