First,the number of all the propagating modes has been calculated by an integral formula already known.Then two integrals have been derived by using WKBJ method to calculate the number of two different parts of modes which are not absorbed by the core of double-clad fiber(DCF).According to the number of non-absorbable modes and the number of all the propagating modes,we have obtained the absorption efficiency of graded-index DCF.Numerical examples have been given.Based on calculated results,how the graded-index parameter of offset distance affects the absorption efficiency has been discussed too.