As a core device of a target detection and imaging system, an infrared detector assembly's space resolution may directly affect the imaging quality of the detection system. Usually, the Modulation Transfer Function (MTF) is used to evaluate the spatial resolution. However, both electric crosstalk and optical crosstalk of a detector assembly are the main factors affecting the MTF. A phase-locked system is used to measure the electric crosstalk of the infrared detector assembly while an infrared micron-spot test system is used to measure the Line Spread Function (LSF) of the assembly so as to evaluate its optical crosstalk. The measurement result shows that the optical crosstalk of the 11.5-12.5 μm waveband detector is obviously greater than that of the 8.0-9.0 μm waveband detector. Finally, the measurement result is analyzed. The analysis result provides a reference for the optical crosstalk design of the infrared detector assembly.