An apparatus for coupling a laser and an optical fiber and an optical signal transmission system and transmission method where the coupling apparatus is disposed between a laser and an optical fiber, where the coupling apparatus includes an optical signal transmission part whose inner refractive index changes gradually, where a refractive index becomes higher at a position closer to a principal axis of the optical signal transmission part; and the optical signal transmission part may be configured to shape an optical signal incident from the laser (including optical signal convergence or divergence), so that a mode field radius of the adjusted optical signal matches a core radius of the optical fiber, and the adjusted optical signal can be coupled into the optical fiber in high efficiency.