A method of estimating the confidence interval for debris population using radar beam fence is described. Whether a debris object may cross the beam fence, that is, whether a debris has large enough inclinationand radar cross section or not, is actually detected and modeled as a (0-1) distribution. Using debris altitude data collected, the average probability of crossing the beam fence is obtained. The confidence interval for debris population is estimated by the Central Limitation Theorem. The validity of the method is verified by simulation.