The latest developments of representative key technologies of low noise CMOS image sensors are summarized. The active pixel structure and the readout circuit architecture of image sensor are introduced from such aspects as the architectures and module design of CMOS image sensors. The readout circuit with in-pixel source follower and its noise equivalent model are analyzed. The key technologies of low noise CMOS image sensor are introduced in detail, including reference-shared in-pixel differential amplifier technique, correlated multiple sampling technique and in-pixel chopping technique, and the circuit implementations of these techniques are introduced as well.