The invention discloses an underwater combined natural gas-liquid separation device and method. The underwater combined natural gas-liquid separation device is characterized by comprising an air inlet line and a plurality of separator single pipes, wherein a plurality of air inlet branch lines are arranged at an outlet end of the air inlet line; each separator single pipe is sequentially partitioned into an exhaust chamber, a liquid collecting chamber, an air inlet chamber and a drainage chamber through a first partition plate, a second partition plate and a third partition plate from top to bottom, and valves are arranged on the first partition plates and the third partition plates separately; a liquid guide pipe for communicating the corresponding liquid collecting chamber and drainage chamber is arranged in each air inlet chamber; a cyclone core tube for communicating the corresponding exhaust chamber and air inlet chamber is arranged in each liquid collecting chamber; and each air inlet branch line on the air inlet line is connected with the air inlet chamber of each separator single pipe.