Abstract: The paper has evaluated the relationships among three terrain reduction procedures(terrain correction,isostatic reduction,residual terrain reduction(RTM) and given the methods of their data processing.The emphasis has been laid to speed the computation and deal with the singular integration properly.Our major conclusions are as follows:——To the near zone of the computated point,or the'innermost zone',the terrain heights have a critical effect on the terrain reduction results,and the discrete integration need to be done particularly.The(terrain) height data in the innermost zone must be densified to decrease discrete error,the best method is bicubic spline function.——It is necessary to obtain detailed terrain height data in the innermost zone.(generally speaking,within 10~50km radius is enough).——It is highly important to consider the speed of computation.If there are many terain reduction points to be computated and the points are regular arranged,FFT algorithm is effective to save computer time.