We describe an optical imaging system based on random phase modulation and sparse sampling. The system is also an application of compressive sensing in the field of imaging. Different from the Rice compressing imaging system, the imaging system needs only a signal exposure. Meanwhile, the space-bandwidth product of the system is extended, which means the pixels of imaging sensor can be reduced without a loss of image's resolution. A random phase modulation is employed to make the energy of optical field spread out across the entire modulated image, which can facilitate the sampling process. A random sparse sampling scheme is designed to effectively reduce the pixels of imaging sensor, and the sensing matrix is uncorrelated with arbitrary sparse representation matrix. The feasibility of the proposed system is validated by numerical experiments.