A timing circuit is designed to reduce the effect of digital noise on analog signal in CMOS image sensor(CIS).Each module of the digital circuits can work time-sharing by adding gated-clocks ware and it can keep quiet to suppress onise when analog circuits are sampling.Theory analysis and experimental results show that the noise at sampling is reduced by 70%,and 20% in other time.The method to keep digital circuit quiet when analog circuits are sampling can be extended to all high-precision mixed-signal integrated circuits design.