The very good method to analyze the structural change of reaction process of UF resin synthesized by the traditional process alkali-acid-alkali is 13C-NMR,and it may extrapolate the possible reaction. The structure analysis indicated that, under alkalinity condition, formaldehyde and urea reacted by addition reaction was primary. The reaction production was mainly monomethylolurea. The urea participated in the reaction completely. The formaldehyde this time still had existence by polymerization, at the same time had the occurrence of polycondensation reaction. At the acid reaction stage the polycondensation reaction was primary and formed the macro molecule system with the methylene or the ether linkage, and the polymerization formaldehyde decomposed completely, and as the form of methylene glycol participated in condensation reaction. The third urea joined may obviously reduce the free formaldehyde content of the reacting system, at the same time the reaction production was mainly monomethylolurea.