The influence of electrolytes on the diffusion properties of polystyrene colloidal particles in dense suspensions is measured by means of phase-modulated fiber optic low-coherence dynamic light scattering. In our experiments, both NaCl and BaCl2 are used as electrolytes. Experiment results show that the diffusion coefficients of the polystyrene colloidal particles increase with the increasing of ion concentration and ion valence when the concentration of electrolyte is below 0.01mol/L and the temperature keeps constant. The diffusion coefficients obtained experimentally show good agreement with the diffusion coefficients obtained by using the Stern model.