With the development of infrared focal plane technology,the infrared focal plane detectors have become widely used,which puts higher requirements on the sensitivity of infrared focal plane detectors.Firstly,In this paper,the noise reduction principle of the traditional TDI readout circuit is firstly analyzed,and the limitations of the traditional TDI readout circuit to improve the sensitivity of the infrared detector are discussed through simulation,testing and theoretical analysis,and the optimal NETD value of the traditional TDI type infrared detector is calculated to be 4.19 mK.Then the noise source of the pixel-level digital TDI readout circuit and the way to reduce various noise are analyzed.Finally the simulation results and theoretical calculations show that the NETD of 32 pixel-ADC TDI can achieve sub-K level,so the readout circuit can achieve the requirements of very high sensitivity infrared detectors.