Presented was an optimum designed CMOS active pixel sensor. In this sensor, used is a PMOSFET substituting for the NMOSFET in traditional sensor as restoration transistor. Compared with traditional active pixel sensor under the same condition based on 0.25 μm CMOS technology, simulating results show that the new structure device has higher signal-to-noise ratio, wider output swing, wider dynamic range and faster readout speed.