Abstract: To lay a theoretical foundation for the determination of a better synthesizing condition, the structure of phenol-melamine-urea-formaldehyde resin (PMUF) synthesized in a typical way and the structure changes during its preparation were examined by ¹³C nuclear magnetic resonance (¹³C-NMR) spectroscopy in this paper. The results showed that the NMR graph became simpler after adding melamine to UF resin. So the formation of methyl ether bridges between melamine and urea was possible. Almost all of the phenol was free in this resin, ie phenol didn't co-condense with UF species, MF species, or even formaldehyde. So the method to add phenol to MUF resin in the end of the preparation of MUF resin couldn't guarantee the co-condensation between MUF resin and phenol to come forth.