An automatic oil pump includes a drive chamber, having a reciprocal driving device, and an oil chamber, having a sucking device, located on two sides of a main body. The reciprocal driving device has a piston unit having a valve chamber at its center and a balance valve and is pushed by a spring. The drive chamber has grooves on its inside wall. The piston unit has two channels, allowing air to flow into the valve chamber past two sides of the piston unit. On the piston unit is a driving rod. The driving rod penetrates into the main body and reaches into the oil chamber. A hermetic cover and a movable piston are attached to an end of the driving rod. On the wall of the piston are pluralities of through holes. The reciprocal driving device drives the piston in the oil chamber to move reciprocally, so oil is sucked into and out of the oil chamber continuously. Thereby, oil sucking speed and volume are upgraded to save time and labor, and oil pumping efficiency is enhanced.