Based upon "box model" of river bed, according to the landform parameter of landform characteristics of stochastic distribution of river bed, this paper studied and deduced the calculating method of duplex linear interpolation by weighted average of altitude-divide-joint of river bed net. And also by using calculation method we computed separately the volume of erosion and siltation according to the cuboid, section and column of the river bed, and evaluated the siltation content and the landform parameter according to river bed joint, and established the digital model of river bed landform and finally drew the isohyet landform map automatically. By applying this method, we carried on the calculation of the siltation content and the autodraft of the landform map of the river bed which was formed by five artificial intermittent crustal movements in the laboratory.