In order to extend the depth of field and improve the reliability of the system without reducing the resolution of the imaging optical system for space targe, this paper proposes and designs an imaging optical system for space target based on wavefront coding technology to extend the depth of field. On the basis of the initial imaging optical system for space target, the cubic phase plate is optimized based on the consistency of the modulation transfer function curves of different object distances and the recoverability of the images. The FOPD algorithm is used to achieve a better intermediate images recovery effect. The effects of the initial optical system and the wavefront coding optical system are compared, and the results show that the depth of focus and depth of field of the wavefront coding optical system have been significantly expanded. The total length of the optical system is 41 mm, the focal length is 20 mm, the working wavelength is 850 nm. The simulation experiment results show that without the need for focusing mechanism, the depth of focus of the system is 28 times that of the initial system, and the depth of field of the system is extended from 0.956~1.100 m to 0.5~130 m. The system can meet the needs of large depth of field imaging without focusing mechanism in the fields of space rendezvous and docking, satellite capture and so on.