A low pressure accumulator for an anti-lock brake system of a vehicle. The low pressure accumulator includes a cylinder having one end communicated with a fluid path connected to a valve and a pump and an opposite end formed with an opening, a piston moving back and forth in the cylinder, a spring installed in the cylinder to elastically support the piston, and a plug installed in the opening of the cylinder to support one end of the spring. The plug includes a vent to ventilate air through an inside and an outside of the cylinder.