The present invention provides an interferometer optical system comprising a light source, a collecting lens, an optical filter, a field diaphragm, a spectroscope, a collimator, a phase shifter, a microscopic objective and a charge-coupling element; light emitted by the light source sequentially passes through the collecting lens, the optical filter, the field diaphragm, the spectroscope, the collimator and the phase shifter to go into the microscopic objective lens and is subjected to light splitting by a light splitting surface of the microscopic objective, so that a part of the light is reflected to a reference reflecting surface of the microscopic objective lens and returned by the reference reflecting surface, and the other part of the light passes through the light splitting surface of the microscopic objective lens to arrive at a measured surface and reflected back by the measured surface; the part of light then passes through the light splitting surface again, intersects with the light reflected by the reference reflection surface, causes interferences, then gets reflected by the spectroscope to enter the charge-coupling element to generate an interference pattern. The interferometer optical system of the invention has a small number of components, a simple structure, and easily guarantees the lens barrel machining precision and reduces adjustment difficulty.