Three-way catalyst (TWC) converters are used to remove harmful substances (e.g., carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HC)) emitted from gasoline engines. However, a large amount of emissions could be emitted before the TWC reaches its light-off temperature during a cold start. For hybrid electric vehicles (HEVs) powered by gasoline engines, the emission purification performance by TWC converters unfortunately deteriorates because of mode switching from engine to battery and vice versa, which can repeatedly generate cold start conditions for the TWCs. In this study, aiming to reduce emissions from series HEVs by early activation of TWCs, numerical simulations and experiments are carried out. An HEV is tested on a chassis dynamometer in the Worldwide Light-duty Test Cycle (WLTC) mode. The upstream and downstream gas conditions of the close-coupled catalyst converter are measured. A test piece is taken from the same catalyst and used in model gas experiments to decide the chemical reaction scheme and each corresponding reaction rate parameter. A 1-D numerical simulation TWC model, which includes 13 chemical species with 22 global reactions, is built using GT-Power, a commercial software by Gamma Technology. The TWC model is able to reproduce the close-coupled TWC’s purification performance of CO, NOx, and HC. Furthermore, an electrically heated catalyst (EHC) model is added to the TWC model, aiming at faster heating up for early catalyst activation during the first warming up period from a cold start. The effects on emission reduction by EHC using a range of input power are investigated. It is found that the oxidation reactions can be activated earlier with the EHC installed; and the emission of CO and HC can be effectively reduced using a certain amount of power that can be sufficiently supplied by the existing lithium-ion battery on the series HEVs.