The aggregation structure of surfactant 2,3-dimethyl-5- (5'-16 alkyl) benzene sulfonate (C16-5OXS) at the oil/water interface was investigated by union atom molecular dynamics simulation. The effect of weak alkaline Na2CO3 on the aggregation behavior of C16-5OXS molecules was also investigated. The results showed that C16-5OXS can form a monolayer at the oil/water interface. With the increase of the concentrations of Na2CO3, the thickness of the interfacial film is decreasing. At low concentrations of Na2CO3, The thickness of the electric double layer of C16-5OXS has little change. But the thickness of the fixed layer sudden drop from 0.106nm to 0.082nm.when the concentrations of Na2CO3 is 1.2 %, at the same time, hydrophilic area decline constantly. C16-5OXS can form a closed packed monolayer at the oil/water interface. The decline of order parameter of hydrophobic chain atoms showed that the conformation of hydrophobic chain will change with the addition of Na2CO3. Research the mechanism of action between Na2CO3 and C16-5OXS at the molecular level.