The invention discloses an automatic reversing water injector. The automatic reversing water injector comprises a water pipe, an air pipe, a pulp pipe, a water jacket pipe, a horizontal water injection nozzle, a vertical water injection nozzle, a piston, a piston bush and a piston spring, wherein the air pipe and the water pipe are sequentially and concentrically arranged at the inner upper part of the pulp pipe, the water jacket pipe is arranged at the inner lower part of the pulp pipe and welded on the inner wall of the pulp pipe through a water and pulp pipe sealing plate, the piston is arranged in the water jacket pipe through the piston bush, the piston spring is sheathed at one end of the piston, two ends of the piston spring respectively prop against the piston and a piston bush sealing plate, the water pipe is welded with the piston bush, the air pipe is welded with a water and air pipe sealing plate, an air nozzle connector and a guniting hole are respectively welded and arranged on the same side of the pulp pipe, the horizontal water injection nozzle is in threaded connection to a horizontal water injection nozzle connector which is connected with the piston bush, an air nozzle is in threaded connection to the air nozzle connector, and the vertical water injection nozzle is in threaded connection to a water jacket pipe sealing plate. Under the action of the water pressure and the piston spring, the piston moves up and down to complete the pore-forming and guniting operations in a reversing way. The automatic reversing water injector is applicable to quicksand formation and solves the difficult problem of pore-forming on the quicksand formation.