The whole-geometric-approximation-method(WGAM) is presented to compute the forward light intensity. Since Fraunhofer diffraction and geometric scattering are combined, such a method is applicable under anomalous or normal diffraction regime. Due to the consideration of interference between the diffraction, refraction and the reflection rays, the calculable angle range is extended to 0°～60°. Because of its high calculation efficiency and wide validity range, WGAM is especially effective for light pattern calculation of the large particles with size parameter α larger than 40. Numerical calculations show that the results of WGAM are consistent with Mie theory. The experiments also find that WGAM can be applied in laser particle sizing based on large-angle forward scattering method.