Combined with Fast Fourier Transform (FFT), a mathematical hybrid method for accurately computing the lightning current flowing along the grounding grid buried in multilayered earth model has been developed in this paper. In the hybrid method, electrical circuits consist with "T" typical elementary element. To accelerate the calculation, both dynamic state and quasi-static complex image methods, and closed form of Green's function were introduced into this model. The model can be used for study performance of transient lightning response to grounding grid.