Super-resolution (SR) imaging is a technology based on oversampling to reduce the aliasing and increase the image resolution. Pixel active area (PAA) model is essential in SR related research. The difference of MTF with the widely used square model and the“Z” shape model we proposed was calculated to simulate the practical infrared focal plane array(IRFPA) production. The cause of the difference was analyzed. Furthermore, a hypothesis was proposed, which the smaller the difference between the closest and the farthest distances from the PAA boundaries to the pixel center, the greater the MTF of IRFPA will be. Different PAA shapes were discussed and the MTFs of these shapes were calculated. The calculation results verify the hypothesis we proposed. These results also show that the circle PAA is the most ideal model and it should be used as a reference for IR FPA design.