The rheological behavior, molecular weight distribution (MWD), reactivity, and chemical structures of UF resin during the pre-curing process were elucidated by rheometry, gel permeation chromatograph (GPC), differential scanning calorimetry (DSC), and C-13 NMR, respectively. The unsynchronized increasing processes between viscosity and rigidity during the pre-curing process were closely related to the changes of activation energy, MWD, and chemical structures. The results indicated that, due to the pre-curing behavior showed significant influence on the activation energy, both the occurrence of pre-curing behavior and its degree could be accurately judged, and the change trend of MWD and the chemical structures of the pre-curing resin could be also predicted by the non-isothermal DSC method. Once the value of Ea was higher than that of E-0, the degree of cross-linking was increased and the network structure was formed indicating that the pre-curing behavior of UF resin had taken place.