In this paper, the beam quality and polarization degree of laser beam which propagated in large core multimode power fibers is studied. Domestic fused silica optical fiber with core diameter of 0.6 mm and 3M silica fibers with core diameters of 0.4 mm and 0.6 mm were used for experiment. It is found that, the output beam quality factor M 2 is a compound tanh function of fiber lengths of every kind fiber. Linearly polarized laser beam launched into a multimode fiber suffer depolarization, the output beam′s polarization degree is an exponent function of fiber length and this relationship is agreed well with every kind of fiber.