A novel algorithm for noise detection and filtering is proposed in this paper based on space-time characteristics of nautical radar data. The noise can be detected and filtrated by the improved median filtering algorithm according to the uncorrelated property among various scans. The algorithm for traditional median filtering and that for mean filtering are analyzed in comparison with the algorithm proposed in this paper，and the evaluation parameter of SNR is utilized to estimate the results. It is shown that the algorithm proposed in this paper preserves not only such advantages of the traditional median filtering as simple calculation and high speed computability but also the gray information of the original image. In addition，a good noise reduction effect can be achieved.