The principle and fabrication technology of the high-concentration Tm3+-doped double clad fiber is reported.The process gives the high Thulium concentrations and the high homogenity of the concentration,with the contract test of the heated-delivery tube temperature and Thulium concentration,the perfection of the research for new fabrication equipment,the proper tube temperature for doping.The new technology avoided the depressed center of perform sucessfully,reduce the background loss of the fiber,the satisfying Thulium-doped double-cladding fiber is fabricated sucessfully.The concentration of Tm-ions is up to 0.6%,and the inner cladding is D-shape.