The porosity (including the specific surface area and pore volume-diameter distribution) of montmorillonitic soft rock (MSR) was studied experimentally with an electrochemical treatment, based on which the change in porosity was further analyzed from the perspective of its electrokinetic potential (${zeta}$ potential) and the isoelectric point of the electric double layer on the surface of the soft rock particles. The variation between the ${zeta}$ potential and porosity was summarized, and used to demonstrate that the properties of softening, degradation in water, swelling, and disintegration of MSR can be modified by electrochemical treatment. The following conclusions were drawn. The specific surface area and total pore volume decreased, whereas the average pore diameter increased after electrochemical modification. The reduction in the specific surface area indicates a reduction in the dispersibility and swelling-shrinking of the clay minerals. After modification, the ${zeta}$ potential of the soft rock was positive in the anodic zone, there was no isoelectric point, and the rock had lost its properties of softening, degradation in water, swelling, and disintegration. The ${zeta}$ potential increased in the intermediate and cathodic zones, the isoelectric point was reduced or unchanged, and the rock properties are reduced. When the ${zeta}$ potential is increased, the specific surface area and the total pore volume were reduced according to the negative exponent law, and the average pore diameter increased according to the exponent law.