PVC (polyvinyl chloride) material products are widely used in all kinds of buildings. When fire happens, PVC material burns easily, and a great deal of HCl (g) is released. HCl (g) turns into acid mist with the presence of water vapor, and coagulate in the surface of concrete. At the same time,it will be quickly absorbed by concrete. When the concentration of chloride ions in the surface layer of the concrete is high, the chloride ions in the cover layer of concrete diffuse into inside of concrete with increasing time, which influences durability of concrete greatly. A mathematical model to describe the activity of chloride ions in the concrete after fire is set, the model is verified to be valid by the related data, and the sensitivity of corresponding coefficients is analysed. A theoretic basis for assessment of the durability of concrete structure after fire is proposed.