Infrared detector module is the key device of target detection and imaging system and its spatial resolution directly affects the imaging quality of detection system. When evaluating spatial resolution of detector modules, researchers usually adopt modulation transfer function (MTF), which is mainly influenced by the optical crosstalk of detector modules. The basic principle and configuration of infrared micron-spot test system, whose optical dispersion diameter is 30 μm are introduced. The line spread function(LSF)of detectors with different structures is measured by the scanning slit technique, and a corresponding model is built for analysis. Theoretical analysis and numerical simulation agree well with the experimental results, demonstrating that the profile of overlap electric region is photosensitive, which is the main factor to cause widened LSF, asymmetry, secondary peak and so on. The result provides a reference for the design of infrared detector modules to suppress light stray.