Based on the geometry optical theory and the diffraction optical theory, the variety of wavefront error with the propagation distance in free space is analyzed, so the spatial conjugation request of deformable mirrors, wavefront sensors and the corrected pupil in the Adaptive Optics (AO) system are presented. The results show that the propagation distance, the aperture of beam and the magnitude of the aberration affect wavefront error together. The wavefront variety is not distinct for small aberrations near propagating with a big aperture. Generally, the system’s compensation capability doesn’t decrease when the wavefront sensors and deformable mirrors depart from the conjugation position in some range whose Fresnel number is more than 1000. The positions of wavefront sensors and deformable mirrors should be designed and the 4F optical system is selected according to the character of the aberrations. This paper will provide some theoretical reference for the optical design of the AO system.