The invention discloses an in-situ calibration method for a computer-generated-hologram substrate etching error. With the method, the computer-generated-hologram optical path position can be kept to be unchanged on the condition of computer-generated-hologram aspheric optical path detection condition so as to realize in-situ calibration of a computer-generated-hologram substrate etching error. The specific optical path structure is formed by replacing a to-be-measured aspheric surface with a spherical mirror; and a compensator is added into the computer-generated-hologram spherical mirror to compensate system spherical aberration. Before etching processing of computer-generated hologram, a parallel flat-plate substrate is placed into the optical path for first-time detection; and after holographic etching processing completion by the computer-generated-hologram substrate, the computer-generated hologram is placed into the optical path for second-time detection; and a difference between two detection results is a calibrated computer-generated-hologram substrate etching error. With the method, an aspheric F-number restriction problem in the existing in-situ calibration method can be solved; and the calibration precision is not limited by the manufacturing precision level of the compensator and the surface shape quality of the to-be-measured spherical surface and thus calibration on the compensator and the surface error of the to-be-measured spherical surface is not needed.