Abstract: The mechanism that hydration ions in the mud to cement slurry (MTC) cementing fluid can diffuse and migrate into the mud cake from multifunctional drilling fluid has been well researched, while it still remains unclear at present why hydration ions in the oil-well cement can not diffuse into the mud cake from multifunctional drilling fluid. Herein, we examined differences in ion compositions of the oil-well cement and the MTC cementing fluid by means of ion chromatography, atomic absorption spectrophotometer and X-ray diffraction, and investigated thoroughly the hindering mechanism that prevent hydration ions in the oil-well cement from diffusing and migrating into the mud cake from multifunctional drilling fluid. The result indicated that Ca²⁺ in the oil-well cement has a mass fraction much higher than that in the MTC cementing fluid and can form CaCO₃, CaSO₄ and Ca(OH)₂ at the interface between the oil-well cement and the mud cake from multifunctional drilling fluid. The mass fraction of Na⁺ in the MTC cementing fluid, however, is much higher than that in the oil-well cement, moreover, Na⁺ can change the ζ potential on the surface of active particles in the mud cake and destroy the water film formed on the surface of organic additives from drilling fluid, which are the major reasons that make hydration ions in the MTC cementing fluid capable of diffusing and migrating into the mud cake from multifunctional drilling fluid. Whereas, the CaCO₃, CaSO₄ and Ca(OH)₂ formed at the interface of the oil-well cement and the mud cake from multifunctional drilling fluid may precipitate and block apertures, which is attributed to the main hindering mechanism that prevents hydration ions in the oil-well cement from diffusing and migrating into the mud cake from multifunctional drilling fluid.