In this paper, the noise of staring thermal imaging system is discussed. Firstly, the differences between the first and secondgeneration thermal image system are stated and analyzed. Secondly, the 3D noise model is introduced, which is used to analyze the noise of the staring image system. The noise of the system is divided into 7 components, and every component is analyzed in detail, the result of the experiment is put forward. At last, the affection of every noise component, which can affect the MRTD, is discussed.