The invention relates to a downhole speed-increase and effect-increase tool under the effect of dual pressure cavities and a method of the tool. Friction resistance is reduced in the drilling processof a deflecting section or horizontal section of a horizontal well, and the drilling speed is increased. According to the technical scheme, the downhole impact speed-increase tool mainly comprises anupper shell, a middle shell, a lower shell, an inlet screw, a piston, a valve assembly shaft and an impact assembly shaft. The speed-increasing method includes the steps that drilling liquid transmitspressure to compress an outer compression spring and pushes an impact nut at the right end to make contact with the piston, the piston drives the impact assembly shaft to move down, and a drill bit connected to the lower portion of the impact assembly shaft to hit rock; when the rock is hit, because the rigidity of an inner compression spring is larger than that of the outer compression spring, the nut and the piston are hit to be separated under the counter-acting force of the inner compression spring, all parts return to the initial positions, and one stroke length is completed. The tool issimple in structure, the friction resistance can be reduced and the drilling speed can be improved, the radial vibration is small, the trajectory is easy to control, the drilling precision is improved, and cost is saved.