In order to improve the output power of thulium-doped double-clad fiber (DCF) lasers, starting from optimizing the absorption efficiency of DCF to the pump light, pump absorption characteristics of fibers with different core diameters and inner cladding shapes are numerically simulated. The simulation results show that the pump absorption efficiency tends to 100% when the reflection times increased for the new inner clad DCFs with different core diameters. For DCFs with different inner cladding shapes, the absorption efficiency of the new inner cladding DCF is the highest. In the experiment, regular hexagonal inner cladding thulium-doped DCF and new inner cladding thulium-doped DCF are selected as the working substances. The output power of the two thulium-doped DCF lasers is 13.2 W and 14.8 W, respectively. The slope efficiency of the two thulium-doped DCF lasers is 31% and 35%, respectively. The output power of the new inner cladding thulium-doped DCF laser is 12.1% higher than that of regular hexagon inner cladding thulium-doped DCF laser, which indicates that the new inner cladding DCF is more conducive to achieve efficient pump absorption.