The invention relates to an NC bending core head for a thin walled tube of aluminum alloy with a major diameter and a design method thereof, wherein the core head comprises an core head(2), a mandrel core pin(1) and a connection member between the core heads(3). Further, the core heads are connected in a ball-and-socket structure manner. The eccentricity delta of the ball centre of the eccentric core (2) in the internal of the core head is delta=h / k, wherein h is the distance between the ball centre of core head and the large end of the core head, and k is the thickness of the core head. The inside radius d0' and the outer radius d0 of the core head, and x2 corresponding to the largest angle theta 2 of the core are determined, so that the relationship between the core distance h and the largest angle theta 2 of the eccentric core is established. Through finite element modeling, the scope of the largest angle theta 2 between the eccentric cores (2) is determined, and furthermore, the value range of h is determined. According to invention, an interference area between rotary core heads, and excess interference between the core heads and the tubing outside arc surfaces are reduced, so as to be suitable for bending and shaping of the tubing under various bending radii. Besides, the possibility of tubing wrinkling and the degree of thinning could be decreased, without increasing the flat degree of tubing.