Alkyl ether carboxylate is one type of surfactant that can produce ultralow interfacial tension (IFT) under high-salinity and high-temperature conditions. In this paper, the influence of counterions on dynamic IFTs of fatty alcohol polyoxyethylene carboxylate (C12EO3C) against alkanes has been studied. The effect of the temperature on the IFT has been investigated. On the basis of our experimental results, one can find that the NaCl concentration has little effect on the IFT, while divalent ions can reduce the IFT to an ultralow value. With the increasing CaCl2 or MgCl2 concentration, dynamic IFT passes through a minimum at a particular salt concentration ("V" shape). Moreover, the stable value of IFT achieves an ultralow value and also passes through a minimum at the same salt concentration. MgCl2 has a stronger tendency to achieve ultralow IFT than that of CaCl2, while the addition of CaCl2 has a stronger tendency to partition surfactant molecules to the oil phase. Ultra low IFT could also be achieved by improving the temperature because of the enhancement of oil solubility of the surfactant. An interfacial model combining two mechanisms, partitioning the surfactant into the oil phase and decreasing the charge repulsive force between interfacial surfactant molecules, responsible for the effect of the electrolyte on dynamic IFT has been provided. All experimental results above can be explained well. Our studies are of great significance in designing ultralow IFT formulation for the reservoir in a high temperature and with high-salinity formation water.