EDF gain shifting to long band with the length increase was demonstrated by analyzing the C-EDFA amplified spontaneous emmission spectrum based on Giles model. Furthermore,transmission characteristic of various beam of light in L-EDFA and C-EDFA was studied and a reasonable explanation for low pump conversion efficiency(PCE)of L-EDFA was provided. Moreover,the intrinsic flat gain property was investigated. Gain spectrum is flat on optimum population inversion level(～40%). Finally,the problem of L-EDFA low PCE was solved by using a fiber loop mirror at the foreside of the L-EDFA to reflect the backward ASE. The PCE increases by 8.76% compared with the conventional structure.