A phase plate is designed to extend the depth of field of the initial optical structure.Several considerations and design requirements for the design of wavefront coding optical imaging system are proposed.According to these matters and the basic parameters selectedthe initial structure of an optical system is designed and optimized by Zemax.The initial structure is an imaging objective lens with an object distance of 4 mF number of 3 and effective focal length of 50 mm.The surface type of the selected cubic phase plate is extended polynomial.By setting a complex evaluation functiontaking the cubic coefficient of the phase plate as a variableand then optimizing the phase plate to obtain its parameters.Finallythrough the numerical comparison between the traditional optical system and the optical system with phase plateit is concluded that the depth of field of the design can be extended from 4 m to the range of 2.5 m to 8 m.