This paper presents theory and method of spectral decomposition in detail,and analyzes the fracture-cavern reservoir with forward model and then generalizes the spectrum's character which depends on the varied fracture-cavern's size and fracture's density.Fracture-cavern's size and fracture's density varies cocurrent with spectrum and inversely with the frequency of spectrum approximately.The technology application suggests that it is effect for carbonate fracture-cavern reservoir.