In order to deeply explore the relationship between detecting point target and scanning device，indicator to measure scanning feature called scanning inhomogeneous was established. According to the physical process of scanning imaging, an imaging model for scanning system with linear detector was established. Based on that model, scanning inhomogeneous affected on TDI detector for point target detecting was analyzed as the core of SNR. Firstly, integration timing order of single detector was set coordinating with the characteristics of scanning structure, and scanning inhomogeneous metric was established in the meanwhile. Then on the basis of the classical imaging model, an imaging model for point target in scanning system was established with the principle of integration order and detector systems. The SNR calculation formula was deduced combining with TDI working principle and detector noise model. With this formula, the performance of detecting point target was analyzed emphatically under conditions of optical systems, target scales and TDI levels. Finally from the view point of ensuring target detection performance, the upper limits of scanning inhomogeneous ratio in optical system was given. The established model reflects the characteristics of scanning detecting system accurately, and the analysis results are significant to guide for system design and index allocation.