On the basis of the traditional magnitude arithmetic of space targets, the computing models of star magnitude of space targets like prism were researched. First, a more detailed arithmetic was deduced to compute effective optical reflective area of space targets like prism by adopting a facet model. For the column could be approximated by prism, the effective optical reflective area of space targets like column was obtained directly. In order to simplify the solving of the model, Satellite Tool Kit (STK) was used to provide relationship among sun、space object and observation station in the magnitude models, and the process of computation was given. The proposed new computing model was verified with other model and some analyses, and the results show that the models are easily solved with STK and the new column model has higher resolution than the experience model.