The good method to analyze the structural changes of reaction process of melamine modified UF resin by the traditional process alkali-acid-alkali is 13C-NMR. It can also extrapolate potential reaction. The structure analysis indicated that, at the beginning of heat preservation, the reaction between urea and formaldehyde was an additive reaction. Main productions are monomethylolurea and dimethylolurea besides less trimethylolurea. The additive reaction between melamine and formaldehyde produced monomethylol-melamine, dimethylol-melamine. At the same time, polycondensation occurred. The polycondensations between monomethylolurea and monomethylolurea, between monomethylol-melamine and monomethylol-melamine, and between monomethylolurea and monomethylol-melamine occurred too. Main connections are methenyl ether linkage and methenyl. From the end of heat preservation to the end of whole reaction, main reaction was polycondensation. In general, there were three types reaction among melamine modified UF resin, urea-formaldehyde system, melamine-formaldehyde system and melamine-urea-formaldehyde system.