This paper has examined how the decenter and tilt of a cubic phase mask plate influence the imaging of a wavefront coding system. The calculated phase term of pupil function with mask decenter and tilt indicates that both decenter and tilt change the shape of the system modulation transfer function in a predictable way by changing the phase and defocus parameters. Simulation in an on-axis three-mirror Cassegrain system is presented to confirm the calculated formula result. Experimental results for mask decenter are also presented. The results demonstrate that the decenter of a phase mask has less effect on the point spread function in the z direction than the x and y directions.