This paper first discusses the transmission of fiber mode field with absorption and then explores the absorption of the pumped light through a concentric-circle-shaped inner cladding.In order to clarify the amplification mechanism for the double-clad Polymer Optical Fiber Amplifier（POFA）

it sets up a numerical model by using the numerical analysis and other methods and analyzes the amplification process and gain characteristics

and calculates the pump absorption coefficient by using the average value and specific function.In addition

it performs the theoretical analysis of the results of the respective acquired gain characteristics.The experimental data and results testify to the feasibility of this method.