Abstract: The theory of boundary reflection is used for arbitrary dots in the interface of the first and second cladding of double-clad fiber (DCF) to confirm the area where the light source emits the rays that can be absor bed by the RE-doped core.Then the absorption efficiency of the dot will be obtained if this area is compared with the whole area of the first cladding.In order to calculate the whole absorption efficiency of DCF,the absorption efficiency of every dot in the interface of the first and second cladding of DCF should be calculated and synthesized.The conclusions of absorption efficiency of circular DCF and offset DCF completely accord with the results calculated by another ray method.However the process of this method is very handy.