Discrete spatial under-sampling effect of the detector array has a great impact on the imaging results, so the quantitative method of the under-sampling effects is a crucial issue for the performance evaluation of an electro-optical imaging system. Considering the human visual characteristics, a target-characteristics-based equivalent blur model for under-sampled imaging systems is proposed. The introduction into the model of the tolerance of the under-sampling system to the spurious response and the effective frequency spectrum makes the newly proposed model more accurate in the prediction of the target discrimination performance of an under-sampled imaging system.