The isothermal differential scanning calorimetry(DSC) method was used to determine the relationship between curing degree and time of a epoxy resin of CYD-128 with GA-327 as curing agent. An AR 2000EX rolling rheometer was used to measure the resin viscosity in a range of isothermal temperatures. Comparing the isothermal conversion-time relation with that of the isothermal viscosity-time, the isochronous relation of the isothermal viscosity-conversion was set up correspondingly. Results suggest that under the individual influence of the chemical mechanism of viscous increase, the viscosity of the resin system slowly increases with the increase of isothermal conversion, and then increases quickly when the conversion reaches a certain extent. Changing the isothermal viscosity-conversion relation to the relation of viscosity-temperature under the constant conversion, it shows that under the individual influence of the physical mechanism of viscous falling, the resin viscosity decreases with the temperature increased, and the viscosity descendent extent of the resin system increases with the conversion increased. These two mechanisms are separated from each other, which is the technical base of the accurate prediction for the reactive resin system.