Abstract: With the effect of the solid surface on the gas molecules in the gas phase plume deposition in mind, the characteristics of physical absorbance and chemical absorbance are ayalyzed, and the absorbance velocity equation, emission velocity equation and deposition velocity equation are obtained. The absorbance layer near the wall and the physical absorbance layer are distinguished and the emission velocity in the physical absorbance layer is studied. The appropriate emission velocity parameters are chosen for the representative gases CO₂, NH₃, H₂O and N₂H₄. The theoretical emission velocity curves of CO₂, NH₃, H₂O and N₂H₄ obtained are found agreeing with the empirical emission velocity curves in literature. And the gas phase plume contamination of the solar arrays from the monopropellant N2H4 catalyzing decomposition attitude control thruster is analyzed.