As the core component of infrared imaging equipment, the performance of the infrared focal plane array (IRFPA) detector directly affects the imaging quality of the equipment. The unavoidable diffraction effect can lead to crosstalk between pixels, seriously affecting the performance of the detector. A crosstalk testing method based on slit optical system is proposed, which uses high-precision slit optical testing equipment to obtain the level data of the detection pixels and their horizontally adjacent pixels within the slit range, and at the same time achieves quantitative evaluation of crosstalk phenomenon. This article uses test data from medium wave 320×256 mercury cadmium telluride (30 μm) detector, medium wave 320×256 mercury cadmium telluride (50 μm) detector, and medium wave 320×256 indium antimonide (30 μm) detector to verify the proposed crosstalk testing method. The verification results demonstrate that the proposed method can accurately describe the crosstalk phenomenon and quantitatively analyze the impact of crosstalk on detector performance. Compared with existing small light point testing methods, the proposed method is easier to focus and obtains more accurate crosstalk quantification results, achieving an improvement in testing efficiency.