The pixel noise is one of the fundamental noises in CMOS pactive pixel sensors. We focuse on the pixel noise of CMOS active pixel sensors with three different 3T pixel structures for reducing noise. The analysis results show that soft reset leads to lower noise by a factor of 2 than normal KTC noise level. Besides, the 1/f noise of PMOS is much lower than that of NMOS, and the 1/f noise will become larger with the decrease of grate area. Based on above analysis, the design and simulation of three different CMOS APS are accomplished. Samples are fabricated by standard 0.5 μm CMOS process. Test results show that the changes of noise with different pixels match well with analysis results.