To investigate dispersion mechanism of water-based ferrofluid, the effects of electrolytes on the dispersibility of ferrofluid in the dispersing system with different pH values were discussed. The ζ-potential of magnetic nano-particles was measured to discover the adsorbent state of oleate group on the surface of magnetite particles. The mechanism that coexisting electrolyte influences the dispersibility was studied. The results show that the electrolyte affects the stability of ferrofluid through an effect on the structure of surfactant bilayer adsorption, which was proved by ζ-potential measured. The associated mechanism of steric and electrostatic is dominant in aqueous ferrofluid.