Sodium loop is one of the most important characteristics in computer-aided diagnosis of iris. It is related to the discretion of the cholesterol content in the body closely. This paper presents a algorithm of iris sodium loop detection under the space of HSI, Firstly, Segmenting the iris with sclera, eyelash and eyelid to complete feature selection, in order to reduce the shade of effects on the detection of sodium. Secondly, do the rectangular expansion process on iris image, to reduce the sodium loop detection range. Finally, under the HSI space, analyzing rectangular iris image's hue and saturation. Through the data fusion of H, S component to complete the sodium loop detection automatically, Experimental results show that the algorithm can achieve accurate detection of sodium loop.