The invention relates to a submarine mud suction system and method for realizing marine riser-free mud reclamation well drilling. The invention is characterized in that: when the system and the method are applied in shallow water, a submarine mud suction device comprises an outer shell, the lower end of the outer shell is connected with a submarine well head by virtue of a hydraulic connector, a side outlet extends outward from the side wall at the lower part of the outer shell, the side wall of the side outlet is provided with a drilling fluid pressure monitoring system; the side outlet is connected with the inlet of a submarine mud lift pump by virtue of a submarine drilling fluid conveying pipeline, the outlet of the submarine mud lift pump is connected with a drilling fluid processing device on a sea drilling ship by virtue of a drilling fluid return pipeline; and the outer shell is provided with a through hole used for being penetrated through by the lower end of a drill rod on the sea drilling ship; when the system and the method are applied in deep water, the upper part of the outer shell of the submarine mud suction device is a two-stage hopper section, and the lower part of the outer shell is a straight section; and the outer shell is internally and hermetically provided with a rotary assembly, the rotary assembly isolates the lower part of the outer shell and the upper part of the outer shell, and a submarine blowout preventer unit is arranged between the lower part of the outer shell and the submarine well head.