The self-imaging effect in a square core fiber has been investigated, and an integrated all-fiber combiner has been proposed based on a large mode area double clad fiber, which can be employed to construct high power coherent beam combining sources in the all-fiber format. The influence of various parameters on beam quality (${\mathit{M}}^2$) and efficiency of the all-fiber coherent beam combiner has been studied numerically, which reveals that the near diffraction-limited laser beam can be achieved. A principle demonstration of the self-imaging effect has been carried out experimentally in a square core fiber, which proves the feasibility of beam combining with the square fiber, and that it is a promising way to develop high power coherent beam combination sources.