In this paper, an approximation method is presented for calculating the wing load envelope and using to size the wing box in wing strength primary design phase. It is found that the triangle weight load distribution is better than other distributions in simulating the wing weight. Furthermore, this paper investigates the characters of critical over loading and centralized loads for large civil aircrafts. An approximation method for calculating the wing load envelope of bending moment and shear force in flight is proposed by combining the Schrenk lift distribution and triangle weight distribution. Then, the proposed method is validated by wind tunnel data of wing shear forces and bending moments.