Effects of the electrical crosstalk on photoresponse for mid-wavelength InSb infrared focal plane arrays have been numerically studied under back-side illumination. In designing the 2D device model, for geometrical variables the mesa depth and the thickness of substrate are the key points, and for physical models the Poisson equation and continuity equations are taken into account. Our work shows that the crosstalk increases monotonously with the increase of mesa depth and the thickness of substrate. And the optimal values can be found to optimize the device structure.