In the binary composite flooding technology，the experiment study on the micro-mechanism of the influence of Ca²⁺ on ultra-low interfacial tension can provide reference for surfactant formulation optimization. By measuring the interfacial characteristic parameters of petroleum sulfonate KPS，such as the solubility product constant K_sp，surface tension，interfacial tension，and partition coefficient，the main conditions for KPS to form ultra-low interfacial tension were clarified. The results showed that the influence of Ca²⁺ on the surface tension of the KPS solution was related to the decrease in the polarity of the micellar microenvironment. For the KPS system prepared with de-calcium water，the interfacial tension（IFT）was two-order-of-magnitude higher than that of calcium-containing solutions，with IFT restoration observed upon reintroducing 40 mg/L Ca²⁺ . Partition coefficient analysis in 0.3% KPS/kerosene systems demonstrateed that 40 mg/L Ca²⁺ induced optimal surfactant distribution（K= 0.101）with molecular minimum occupied area（A_min）of 0.58 nm². At this time，the sum of the concentrations of KPS in the two phases was less than the total concentration，and the“excess”KPS molecules were concentrated in the interfacial layer. A novel "interfacial complexation" model was proposed，suggesting that Ca²⁺ bridged two sulfonate groups（—SO₃²）near the oil-water interface，which was further validated through disodium phthalate solution experiments. The“interfacial complexation”model can provide molecular-level insights for understanding the microscopic mechanism of calcium ions affecting the interfacial tension of the oil displacement system.