The noise of wavefront sensor is a major noise source in adaptive optics system. Based on the infrared adaptive optics system at 2.16 m telescope, the photon counting noise of wavefront sensor which is photon counting shearing interferometer, and error propagation in the course of close loop are analysed in terms with servo control system. The noise power spectra, servo transfer function, the open loop and closed loop RMS noise error are deduced theoretically. The experimental results are presented.