Iris location is a basal procession in an iris recognition system. The speed and accuracy of the iris location decide the performance of the iris recognition system. In order to speedup the rate of iris location, this paper introduces a fast location algorithm for pupil detecting based on the geometrical characteristics of a circle. By using the coupling relationship between the inner and outer edges of the iris image，a calculus method is used to greatly reduce the search range to locate outer boundaries. Experiments show that the algorithm is fast，accurate and robust compared with other classical methods.