Without dying and fluorescently labeled markers on tissues, Intrinsic optical signals (IOSs) detection technology is a promising method to detect retinal function without influence on normal activities of organism. Slow IOSs of retina reflect the activities in inner retina and the middle layer of retina. In this paper, an IOSs’ detection system was built to detect slow IOSs based clinical instrument, Heidelberg Retina Angiograph 2(HAR2). The retina was illuminated by near infrared light continuously during the experiments. The visible light was presented to stimulate the retina. After image alignment and image processing, the signal-to-noise ratio of the IOSs was improved and the signal changes can be extracted. The experimental results showed that the present detection system is suitable to obtain the slow IOSs in clinic and IOSs detection technology is an effective noninvasive method to study changes of function of retina.