In this paper, we tried to find the relation between the deflection angle of light in the geometrical optic approximation of electromagnetic wave and wave vector according to the quantum theory of light, the energy conservation principle, and the weak equivalence principle. Using the electromagnetic wave equation in gravitational field, we give a general method of calculating the deflection of light by gravitational field under the condition of weak field approximation. Finally, the deflection angles in Schwarzchild metric and Kerr-Newman metric are calculated.