A liquid bubble separator of a circulating water system contains: a cylindrical tank, a pump, a bubble collection tank, and at least one micro bubble generating unit. The cylindrical tank included a water chamber defined by an inner wall, an inlet pipe, an outlet pipe, at least one air orifice, and a bubble discharging hole. The pump is coupled with the inlet pipe to pump treated water into the inlet pipe via the nitrification tank. The bubble collection tank is mounted on one side of the cylindrical tank and includes a collecting tube. Each micro bubble generating unit is disposed in the water chamber and includes a body, a first mixing room, an inlet, at least one air tube coupling with the at least one air orifice and corresponding to the inlet, and a plurality of backflow apertures formed around the body and communicating with the first mixing room.