Model CL256 MOS Image Sensor is a newly developed solid image sensor, called self-scanned photodiode array(SSPD)abroad. Silicon gate P-MOS technique has been adopted in developing the device ,and a kind of three FETS dynamic circuit with varactor bootstrap has been adopted in developing the shift register used as scanned circuit. The circuit has the advantages of high speed and low power consumption. It is very important in developing high-oder digit SSPD devices.