The invention discloses an upward boarding platform and method based on the water surface. The upward boarding platform comprises a lift car system which is arranged on one side of a ship body; the lift car system comprises a lift car guide rail; an extension ladder is arranged in the position, corresponding to the lift car system, of the outer side of the ship body; the rear end of the extension ladder is hinged to the ship body; the front end of the extension ladder is hinged to a buoyancy tank; a first pressure sensor is arranged at the bottom of the buoyancy tank; a metal rod is arranged on the lift car guide rail; the front end of the metal rod stretches towards the water surface and is provided with a pulley and a buffer; a second pressure sensor is arranged in the buffer; the rear end of the metal rod stretches towards the ship body and is provided with a traction motor; one end of a steel wire rope is wound on the traction motor; the other end of the steel wire rope is wound on the hinged portion of the buoyancy tank and the extension ladder; the middle portion of the steel wire rope is wound on the pulley; and a panel layer is arranged on the side, close to the extension ladder, of the bottom of the lift car system. The upward boarding platform can adapt to the change, caused by water level fluctuating and dead-weight change of the ship, of the distance between an elevator and the water surface automatically, and people can board safely and reliably from the water surface and a low pier.