The invention belongs to the technical field of gear engaging gap adjusting, and specifically provides a method for adjusting an engaging gap of bevel gears. The method comprises the steps of 1) connecting the bevel gear I and a rotating shaft through a flat key; enabling the bevel gear I to engage with a bevel gear II; arranging the bevel gear I to be close to the bevel gear II until zero engaging gap is realized; and adjusting the fitting gap between the rear end of the flat key and a shaft key groove to be zero; 2) connecting the bevel gear I and the flat key through a jackscrew; 3) removing set thickness of a semi-cylindrical surface of the rear end of the flat key; and 4) re-assembling the flat key, the bevel gear I and the rotating shaft, thus obtaining accurate engaging gap betweenthe bevel gear I and the bevel gear II. With the adoption of the method, the engaging gaps of the bevel gears can be accurately adjusted without mounting reference.