Abstract: For the atmospheric density near the planetary surface on the planet fixed system or on a spacecraft in the same place at the same time, different measured data are usually obtained. If we want to compare these two sets of measured data, one must convert the measured data from the spacecraft coordinate system to the planet coordinate system through a transformation. On the basis of the theory and the technology of the space vacuum measurements, this paper has obtained the corresponding transformation formulas and validated them. The formulas for the molecular flow density and the atmospheric pressure on spacecraft are derived. The simulation theory for calibrating the directed molecular flow density and pressure is established. Thus, a complete theoretical system for space vacuum measurements is formed. Based on this theoretical system, some recommendations are made for the space vacuum measurement program.