Disclosed a method for manufacturing an oxide layer, applicable to a manufacture procedure of a field oxide layer of a CMOS transistor in the field of semiconductor manufacturing, the method includes: injecting a first gas satisfying a first predetermined condition into a processing furnace in which a first CMOS transistor semi-finished product formed with an N-well and a P-well is placed, and dry-oxidizing the first CMOS transistor semi-finished product into a second CMOS transistor semi-finished product; and injecting a second gas satisfying a second predetermined condition different from the first predetermined condition into the processing furnace, and wet-oxidizing the second CMOS transistor semi-finished product into a third CMOS transistor semi-finished product.