Fast and efficient access to spectral characteristics of multi-cascaded fiber grating Fabry-Perot (F-P) cavity, is an important foundation and premise to ensure optimizing the design of cascade multi-wavelength lasers, amplifiers and other optical devices, and complex distributed sensing network based on the F-P structure. V-I transmission matrix method is presented first in this paper to analyze the reflection spectral characteristics of fiber grating F-P cavity and then the V-I transmission matrix model is established. Spectral characteristics of three different structures of fiber grating F-P cavity at different parameters are analyzed based on the V-I transmission matrix model. Compared with multi-layer method, V-I transmission matrix method can save the computation time apparently under the premise of ensuring analysis accuracy. The experimental results show that for analyzing the spectral characteristics of fiber grating F-P cavity, the V-I transmission matrix method is more accurate than the coupled-mode method.