The design of a combined functional phase-only diffractive optical element, which can be utilized in intersatellite optical communication, is demonstrated. The adopted design method of this paper is based on the nonlinear phase transformation by the law of colour separation, Rayleigth-Sommerfield diffraction theory, direct search optimization method, and Genetic algorithm. This combined functional phase-only diffractive optical element (CPDOE) is motivated by the intersatellite optical communication (IOC). To simplify the structure of optical systems in IOC, the PDOE combined two different functions into the same one element. The two functions are the colour separation and spatial focusing of beacon laser and signal laser. With the utilization of this element the structure of IOC can be highly simplified and integrated. The motivation of being used in IOC also raises the demands of large numerical aperture, polarization insensitivity and high diffraction efficiency. In order to predict the large angle diffraction precisely, the diffraction between the back side of DOE and focusing plane is computed by the means of Rayleigth-Sommerfield (RS) diffraction theory, a more rigorous diffraction theory than Fresnel diffraction theory. To get a high performance of optimization, the direct search and Genetic algorithm has been adopted, and the initial phase value has also be synthesized by the nonlinear phase transform by law of colour separation. The presented design result showed a high performance of the CPDOE. The high performance CPDOE will play an important rule in IOC to increase communication rate as well as decrease volume and weight.