Abstract: A new method of inversion equivalent complex permittivity is explored in this paper based on flat-reflection method and the conditions-selection curve which has homology with the non-reflective curves.The research shows that the results between theoretical simulation by choosing the points on non-reflective curve and experimental of the absorption curve of different thickness samples are all in good agreement.This study provides an effective method for testing the equivalent permittivity of a variety of large size,porous materials,liquid samples etc,which has an important significance for the design of electromagnetic functional composite material with large multiphase internal structure.