1. A method for numerical control milling and forming of a large-diameter aspheric lens, the aspheric lens having a concave surface, with a generatrix equation being denoted as f1, the aspheric lens having a diameter of D, and a numerical control machine tool for milling having positioning accuracy of β, wherein an aspheric surface of the aspheric lens is discretized into a series of rings with different radii, the rings are equally spaced, there are a total of N rings, the rings each have a width of dx, and the rings are sequentially machined through generating cutting by using an annular grinding wheel tool with a diameter greater than a semi-diameter of the aspheric surface; a process of solving the ring width dx is as follows: a generatrix equation of a (N−1)^th ring is denoted as f2, a generatrix equation of an N^th ring is denoted as f3, an intersection of f1 and f3 is denoted as (x1, z1), an intersection of f1 and f2 is denoted as (x2, z2), an intersection of f2 and f3 is denoted as (x3, z3), a point on the generatrix equation f1 of the aspheric lens at x3 is denoted as (x3, z4), wherein x1=D/2, z4−z3=β, x2=x1−dx, and dx is calculated from the geometric relationship.