The infrared spot detection is the key technique used in optical measurement and detection instruments, such as infrared automation optometry, infrared range finder. The precision and speed of the detection algorithm affect those of the optical measurement system directly. The detection system is currently based on PC, with the question of real-time and stability. There is a way to realize realtime detec-tion for low SNR infrared spot using FPGA based on gravity model. In test circuit, firstly,analog CCD video signal is converted to CCIR656 digital video signal using the video decoder chip SAA7113; secondly, in FPGA, a streamline structure is used for histogram calculating, binary image, five times eroding and five times dilating for filtering noise, then the coordinate of centre of cavity is calculated. The test circuit can finish the spot detection in one image in 1/25 s for PAL infrared spots video signal from infrared automation optometry, which was generated by retina reflecting. It requires about 1 s for finishing the above process for one normal PC. The scheme based on FPGA is presented.