Abstract: The inverter-based quasi-static shift register in CMOS APS, which is used in ionizing radiation environment, is susceptible to single event upset (SEU), thus affecting the CMOS active pixel sensor (APS) working. The analysis of the SEU for inverter-based quasi-static shift register concludes that the most sensitive node to single event transient (SET) exists in the input of inverter, and the threshold voltage and capacitance of input node of inverter determine the capability of anti-SEU. A new method was proposed, which replaced the inverter with Schmitt trigger in shift register. Because there is a hysteresis on voltage transfer characteristic of Schmitt trigger,there is high flip threshold, thus better capability of anti-SEU can be achieved. Simulation results show that the anti-SEU capability of Schmitt trigger is 10 times more than that of inverter.