This application discloses a new type of suction leg, an offshore caisson, a sit-on-bottom supporting platform. The suction leg includes a sealing long pile, this sealing long pile including a tubular pipe and a top head connected tightly to the tubular pipe to form cylindrical integral structure with sealing top and opening bottom. The top head has at least one opening to be able to open or close. The sealing long pile can be penetrated into the seabed by a gravity penetration method or/and a suction pile penetration method, or pulled out from the seabed by a buoyancy uplift method or/and a suction pile uplift method.