The influences of nonionic surfactant on interfacial properties of anionic–nonionic surfactant against alkanes are studied by measuring the dynamic interfacial tension (IFT). The surfactants chosen are fatty alcohol polyoxyethylene carboxylate (C₁₂EO₃C) and fatty alcohol polyoxyethylene (C₁₂E₃) with various counterions, Na⁺, Mg²⁺, and Ca²⁺. On the basis of our experimental results, one can find that the addition of nonionic surfactants C₁₂E₃ could achieve an ultralow IFT value at some specific alkane carbon numbers under NaCl condition and the obvious synergistic effect on reducing IFT can be observed during all experimental alkane carbon numbers. However, for MgCl₂ systems, the ultralow IFT values can be achieved by pure C₁₂EO₃C solutions and one can observe synergistic effect and antagonistic effect on IFTs at higher and lower alkane carbon numbers respectively by adding nonionic surfactant. On the other hand, for CaCl₂systems, the addition of 0.05% C₁₂E₃ shows the same influence on the IFTs as that of NaCl systems, while the similar effect as that of MgCl₂ systems will be observed when adding 0.1% C₁₂E₃. An interfacial model combined two mechanisms, controlling hydrophilic–lipophilic balance and forming mixed adsorption film, responsible for varying IFT has been provided based on the difference of ion radius. By adding nonionic surfactant, the mechanisms of forming mixed adsorption film and controlling hydrophilic–lipophilic balance play crucial roles in effecting IFTs for NaCl systems and MgCl₂ systems, respectively. Moreover, for CaCl₂systems, the responsible mechanism of forming mixed adsorption film will change into controlling hydrophilic–lipophilic balance with the increase of nonionic surfactant concentration.