Variable helix angle indexable shallow hole drill and drilling method

The invention discloses a variable-helix-angle indexable shallow hole drill and a drilling method. The periphery of a cutter bar is of a variable-helix-angle structure with the helix angle gradually reduced from the top of the cutter bar to the root of the cutter bar. The inner blade is provided with a radial center passing distance and a radial center passing amount; the radial surface of the outer blade is used as a reference surface, and an axial corner is arranged between the inner blade and the outer blade; the cooling holes are distributed in a Y shape from one cooling hole in the cutter tail and extend to the top of the cutter bar to form two cooling holes, and one cooling hole in the cutter tail is communicated with the two cooling holes in the top of the cutter bar. The depth of the hole is divided into different feeding sections according to the double diameter of the cutter, feeding is stopped when the cutter head reaches the junction of the feeding sections, the cutter continues to rotate at the s