It is extremely difficult to obtain the amount of catalyst in artificial rain dispersal. This paper is trying to estimate the amount of catalyst in artificial rain dispersal from another method, that is, by calculating the liquid water from echoes below-5 ℃ detected by weather radar， and dividing the liquid water into a number of small diameter raindrops (such as raindrops with a diameter of 100 μm), then the maximum and minimum doses of AgI needed in artificial rain dispersal can be gained. Based on the algorithm above, the rain dispersal operation command system of the CINRAD/SA Doppler Weather Radar in the south suburbs of Beijing is developed, which can give the maximum and minimum AgI doses in each grid of the radar detection area with a radius of 230 km，and providing the reference amount of catalyst when carrying out rain suppression.