Abstract: For the extended DLVO theory,the surface potential is difficult to be measured. In this paper,the feasibility of using Zeta potential instead of surface potential to calculate the electrostatic potential was discussed. The surface po- tential of montmorillonite was calculated,and the critical hardness of montmorillonite suspension was calculated by Ze- ta potential and surface potential based on the extended DLVO theory. The results show that:① The water and electro- lyte ions between the surface of montmorillonite particles and the shear surface can shield the surface potential,and the electrostatic potential energy calculated from the surface potentials does not reflect the potential energy of interaction between particles. The electrostatic potential energy of particles can be calculated by Zeta potential instead of surface potential,and the result is in good agreement with the experimental results. ② When the interparticle shear layer starts to contact,the hydration energy begins to increase. Electrolyte ions not only reduce the electrostatic repulsion between particles,but also reduce the thickness of the sliding plane,reduce the distance of hydration,the particles are more easily accessible and coagulation occurs. ③ With the increase of electrolyte concentration,the fluctuation of surface potential is much larger than the change of Zeta potential. The thickness of double layers and shear layer decreases ex- ponentially.