A method of processing a gear reduction box is disclosed. Taking the outer wall of the driving gear hole on the gear reduction box as a reference, scribing the center O2 of the driving gear hole, and scribing the center O1 of the driven gear hole, then extending the connecting line O2O1 to two ends A and B of the gear reduction box body, and punching sample holes to facilitate alignment with a scribing needle. The gear reduction box is hanged on a processing jig, and the gear box is rotated as a whole by left-right movement of the left adjustable support assembly and up-down movement of the right adjustable support assembly, thereby the center O2 of the driving gear hole and the center O1 of the driven gear hole form a horizontal line. The connecting line O2O1 and a work table of the machine tool are horizontal.