For a class of modified terpolymerization system in which the ratio of two primary monomers is kept constant and a trace or small amount of third monomer is involved

the two primary monomers are looked upon as one monomer species

and then the system is treated as a pseudo-binary copolymerization of the "primary monomer" with the third monomer. By kinetic concept derivation method and direct derivation method from simple terpolymerization formula

the formulae for calculating the equivalent reactivity ratios of pseudo-binary copolymerization are derived respectively

and the results obtained by the two methods are consistent. Some terpolymerization examples reported in literature are calculated and the correctness of the proposed method is verified. In this way

not only the calculation of such terpolymerization composition is simplified

but also the selection of the third monomer can be guided

the reactor model can be established

the content change of the third monomer can be predicted

and the reasonable addition method can be guided. It is also found that the terpolymerization based on simple steady state assumption has the property that it can be treated by pseudobinarization.