The behavior in the interfacial polycondensation of 4,4′-methylene dianiline (MDA) and ethylene diamine (EDA) in water with 1,2,4,5-benzenetetra acyl chloride (BTAC) in dichloromethane with tetrabutyl ammonium chloride (TBAC) as the catalyst was examined with ATR,1H NMR, a pH meter, and ESCA. As the BTAC is a tetrafunctional compound with high reactivity, the interfacial polycondensation reaction of BTAC and the diamine is a complicated reaction involving the competitive reaction of hydrolysis and amidation. ATR was employed for analyzing the change in the chemical structure on both sides of the interfacial layer during the reaction. A clear description of the reaction process was made. The interface reaction of this system takes place in the organic phase, and the reaction is diffusion-controlled. It is clear that the film grows toward the organic phase from the interface region until the reaction is over. When sodium carbonate or TBAC is added, the chemical structure of the top and bottom sides formed by interfacial polycondensation is quite different. TBAC assists the transfer of diamine from the water phase to the organic phase, and once TBAC links with diamine, the presence of hydrochloride will not damage their linkage.