In the electro-chemical reinforcement method, the type of injecting solutions has been one of the main research issues. The injection of chemical solution has a significant impact on the electrokinetic properties of the soil, and the main influential factors include the types of cation, ion concentration, cation valence, cation radius, etc. of the injecting solutions. The change of the electrokinetic properties of the soil has a non-negligible impact on the electro-osmotic reinforcement effect. In order to study the effect of cation radius in injecting chemical solutions on the effect of electro-chemical reinforcement of soft soil foundation, taking the divalent metal ions Ca²⁺, Mg²⁺， Cu²⁺ with different radii of the same valence as research objects, the changes of current, drainage volume, electric permeability coefficient during the test and the distribution of water content and shear strength after test were analyzed, combined with scanning electron microscope(SEM) to explore the effect of different radii of cations on the electro-chemical reinforcement effect. The results show that increasing the ionic radius can significantly improve the current, drainage rate, electric permeability coefficient and shear strength of the soil after treatment. Among them, the cation radius of Ca²⁺ is the greatest, and the reinforcement effect of calcium ions is the best. The SEM images also show that soil compactness is higher after calcium ion treatment.