Abstract: In order to improve the splicing quality of large mode area double-cladding fiber, an optical power alignment system was designed by using NUFERN 20/400μm double-cladding fiber. The cladding light in the cladding layer and the basic-mode light in the core were verified. The relationship between core dislocation and coupling efficiency was analyzed theoretically and verified experimentally. The existence of the cladding light or the higher-order mode in the core reduced the sensitivity of coupling efficiency to core dislocation. After stripping the cladding light and the higher-order mode, the coupling efficiency varies with core dislocation as Gaussian shape. A double-ended pumping fiber laser system with kW level output power was built by using the optical power alignment system, with the maximum output power of 1170W, optical-to-optical conversion efficiency of about 73% and beam quality factor of around 1.22. Quasi-single-mode output of kW level was gotten. The results show that accurate alignment of double-caldding fiber can be achieved with optical power alignment technique. The study is important for the improvement of output performance of high power fiber lasers.