The opposite changing trends of surface charge of three variable charge soils to the zeta potentials of the colloids of the soils with ionic strength were observed in this paper. Ion activity and distribution of counter-ions in diffuse layer and shear plane on the soil colloids with changing ionic strength were used to interpret the phenomena. When pH was greater than the point of zero salt effect (PZSE) of the soils, the increasing ionic strength made negative surface charge of the soils more negative and increased counter-cations in diffuse layer and shear plane of the soil colloids and thus made the zeta potential of the soil colloids less negative. On the other hand, when pH was less than the PZSE of the soils, the reverse processes led to the decrease of the zeta potential of the soil colloids with increasing ionic strength. The zeta potential of the soil colloids in KNO3 was less negative than that in NaNO3 due to the greater affinity of the soil surface to K+ than Na+, thus more K+ distributed in shear plane of the diffusion layers of the soil colloids than Na+ at the same concentration of their nitrate.