Iris recognition is the most reliable method in personal identification. However, the current fixed-focus iris imaging system has small depth of field (DOF), which limits the wide application of the iris recognition system. This paper presents the design method and optimization of a phase mask based iris imaging system. Through wavefront coding, it can extend the DOF and enhance the convenience of iris image acquisition. Through analyzing the modulation transfer function and optical parameters of the cubic phase mask, we can get the wavefront coding iris imaging system’s optimal parameter and it’s structure. Experimental results show that the cubic phase mask based iris imaging system has larger DOF and better imaging performance.