The aggregation behavior of N-Dodecyl-N, N-Dimethy–3–Ammonio–1– Propanesulfonate(SB12-3)/sodium dodecyl benzene sulfonate(SDBS) surfactant mixed system at oil/water interface was studied by molecular dynamics simulation. The effects of Ca2 on the amphipathic nature of SB12-3/SDBS surfactant mixed system were investigated at the microscopic point of view. The simulation results showed that SDBS and SB12-3 exhibited the best synergistic effects as the proportion of them reached 4:6. Ca2 replaced the position of Na at the oil/water interface, and compressed the headgroups of SB12-3 and SDBS so that both the surfactants were arranged more closely at oil/water interface. Radial distribution function indicated that the interaction between -SO3- in SB12-3 and SDBS and water molecules increased with the addition of Ca2 , Na were closer to -SO3- in SB12-3, and the thickness of electric double layer was compressed. The solvent accessible surface areas also declined obviously when Ca2 existed in the solution. By calculating order parameter and angles between the hydrophilic groups and hydrocarbon chains of SDBS and SB12-3, it was found that Ca2 could enhance the interaction between decane and hydrocarbon chains of SB12-3 and SDBS, and reduce the oil/water interfacial tension.