A quartz crystal microbalance with dissipation monitoring (QCM-D) combined with a self-made PVDF-coated sensor crystal was used to investigate the deposition and adsorption behavior of BSA on the PVDF surface and the structure of the BSA adsorption the layers at ionic strengths of 0and 100mmol/L, of LiCl, NaCl, KCl. These results were combined with t fouling experiments and the flux recovery rate (at corresponding ionic strengths) to give a deep insight into the effect mechanism of monovalent cation on protein fouling of ultrafiltration membrane. The cause and influence factors of the action of hydrated ion in the process of pollution were described in a microcosmic view. Results showed that, compared with ionic strength 0mmol/L, the hydration repulsive forces between PVDF membrane and BSA or between BSA and BSA could produce effectively by Li⁺, Na⁺ or K⁺, which leading to decrease in the adsorption amount and rate of BSA on PVDF surface, more nonrigid and soft BSA layer was formed and accompanied by a decrease in membrane fouling. However, membrane fouling rate were different at Li⁺, Na⁺ and K⁺ environment. The smaller radiu of ion, the slower adsorption rate of BSA on PVDF surface, the more loosely BSA layer was formed, and lesser membrane fouling was occured. These results indicated that there was a positive relationship between membrane fouling rate and the sizes of ions, which mainly due to the difference of hydrated ability of Li⁺, Na⁺ and K⁺.