An adjusting cap for a mounting axis of an immersed tube method tunnel tube joint comprises two parts including a top plate and a side plate, wherein the outer contact surface of the top plate is a spherical surface and has glossiness, and the periphery of the outer contact surface of the top plate is ground into a rounded corner; the side plate adopts an annular structure with height and thickness, and the thickness of the side plate can be adjusted according to a measured axis deviation of an immersed tube joint; the top plate and the side plate are welded into a whole through single-side groove welding, and a welding seam is ground to be flush with the outer surface of the side plate. According to the adjusting cap, the thickness of the side plate of the adjusting cap can be adjusted in a target manner according to the actual axis deviation of the mounted tube joint; meanwhile, the adjustment effect caused by the adjusting cap is overall displacement adjustment of the tube joint, so that the stress on a GINA rubber water-stop of the joint is not affected, and the effect on the durability of a main structure of a project can be avoided; and the adjusting cap is simple in structure, low in cost and small in construction difficulty.