Proposes an improved algorithm for detecting circles which is based on randomized Hough transform. It use gradient direction information to determine whether the parameter should be accumulated or not. And after obtaining the real circle, update the fringe points muster with deleting parameter λ. The problem of useless accumulation in traditional RHT is well is well solved and improved the probability of the best according circles appearance. Tests on normal images had been taken,and this algorithm has been proved an efficient method.