To analyze the effect of Ca²⁺ ions on the membrane fouling behavior in the mixture of SiO₂ and bovine serum albumin (BSA), atomic force microscope (AFM) technique was used to measure the interaction between foulant-membrane and foulant-foulant under different Ca²⁺ ion contents. Combined with PVDF membrane fouling and cleaning experiments, effects of Ca²⁺ ions on the membrane fouling behavior in the mixed SiO₂-BSA system were investigated. Results showed that with the increase of Ca²⁺ ion content, the membrane fouling caused by SiO₂-BSA gradually decreased, owing to that the Ca²⁺ ions changed the interaction between the foulant-membrane and the foulant-foulant. The addition of the Ca²⁺ ions could effectively trigger the hydration repulsion between PVDF membrane and BSA or SiO₂, which weakened the adsorption rate of BSA on the surface of PVDF and ultimately reduced the membrane fouling rate at the initial stage of operation. As the Ca²⁺ ion content increased, the interaction force between SiO₂-SiO₂ and SiO₂-BSA increased, resulting in the formation of large aggregates and a loose porous fouling layer on the surface of the membrane, accompanied by minor membrane fouling.