To describe and analyze the system level noises of the thermal-mechanical optical readout uncooled infrared imaging system, a photoelectricity coupling model is presented. The photoelectricity coupling device in the model shows the noise isolation characteristic of the optical readout IR imaging system. Based on the model, the noises are partitioned into two kinds: internal noises which are from the Focal Plane Array (FPA), and external noises which are from the optical readout system. The Noise-equivalent Temperature Difference (NETD) caused by internal noises is calculated to be 5.94 mK theoretically. Experiment shows that the NETD caused by external noises is 98 mK, which is close to the NETD caused by the whole noises of the system. The results indicate that the calculation of NETD caused by the internal noises is reasonable and the primary noise problem of the IR imaging system is from the optical readout system