A novel doubleclad fiber (DCF) which has a helix cross section structure of inner cladding and a new method to analyze the absorption efficiency of DCF are brought forward. The theory of probability and the ray method are used to calculate the direct absorption probability of ray which was reflected by the boundary of inner cladding. The probability is used to evaluate the influence of the cross section shape of inner cladding toward the absorption efficiency of DCF. Finally the helix doubleclad fiber is discussed by using this method, and the conclusion is that it is better than other truncated inner cladding DCFs.