With the worldwide strengthening of environmental regulations for automobiles in recent years, series hybrid electric vehicles (SHEVs) have been developed as a promising system that can satisfy both fuel efficiency and power performance. The exhaust gas from SHEV engines is distinctive and different from those of conventional gasoline and parallel hybrid electric vehicles because there is a tendency for repeated intermittent steady-state engine operation at a specific high-efficiency point and engine stop. Consequently, this study aimed at constructing a catalyst model that can reproduce the purification performance on a close-coupled three-way catalyst (cc-TWC) against the distinctive emissions of SHEVs. The emission properties of a commercially produced SHEV were investigated through transient mode tests on a chassis dynamometer. This revealed that the exhaust conditions turn dynamically from lean to rich to stoichiometric over a short period every time the engine restarts, and this behavior is repeated dozens of times per worldwide light-duty test cycle mode procedure. Subsequently, a one-dimensional (1-D) numerical simulation model for the cc-TWC was developed, following model gas experiments under several conditions spanning from lean to rich, to determine the catalyst properties in detail. Emission purification behaviors at the engine restart were reproduced by improving the TWC model to express the reaction rate variation and air excess ratio fluctuation.