The key to realize very high sensitivity infrared detector is to obtain as much charge storage capacity as possible in the limited pixel area of readout circuit. The pixel-level ADC based on pulse frequency modulation is the main method to realize the readout circuit of very sensitive infrared detector. The principle of pixel-level pulse frequency modulation ADC was described. The progress of pixel-level digital readout circuit in MIT Lincoln Laboratory of the USA and CEA-LETI of France were introduced. As a new technology of expanding circuit density from three-dimensional space, the progress of 3D readout circuit was introduced. Finally, the development of readout circuit for very high sensitivity infrared detector in Kunming Institute of Physics was introduced. Using pixel-level ADC technology and digital domain TDI technology, the long-wave 512×8 digital TDI infrared detector assembly was developed by Kunming Institute of Physics with the peak sensitivity of 1.5 mK.