Abstract: Residual fixed pattern noise (RFPN) is becoming a major limitation for system recognition capability due to the continuous decrease of temporal NETD in thermal imager. Based on the analysis of non-uniformity mechanism in thermal imager, a method to achieve scene averaged irradiance in scanning thermal imager is introduced and the scene averaged irradiance is used as a reference to implement dynamic correction. The achieved system RFPN is less than 15mK in the scene dynamic range of (0～50)℃. The prototype was field tested in various environmental conditions. It was proved that such design enables thermal imager performance immune to the temperature change of thermal imager as well as the change of scene irradiance.