Limited by diffraction effect, the size of focused IR spot is usually larger than the pixel pitch of FPAs, hence the crosstalk testing is quite difficult. A crosstalk experimental setup of infrared focal plane array (IRFPA) is introduced briefly. Estimating methods of IR spot size are studied. Furthermore a new crosstalk measuring method called “spot shifting method” is presented. Finally, a determining method of spot shifting distance is given. The focused spot is controlled to scan over the center of an interested pixel along the row (or column) of the FPA. At the same time the output signal of the interested pixel and the spot position are recorded. With these, the spot size is estimated. By centering the focused spot on the interested pixel then shifting the spot to left for a proper distance, the crosstalk of the irradiated pixel to right adjacent pixel is measured. After a similar shifting, crosstalk to other adjacent pixel is also measured. It is vital to select a proper shifting distance. With this spot shifting method, crosstalk is measured even when the pixel size of FPA is smaller than the spot size.