The hot deformation behaviour and interfacial microstructure evolution have a critical influence on the hot workability and mechanical properties of bimetal composite. This study investigated the flow performance and interfacial characteristics of 2205 duplex stainless steel/AH36 low carbon steel bimetal composite (2205/AH36 BC) at elevated temperatures, based on the hot tensile tests over the temperature range of 950–1250 °C and strain rate range of 0.01–1 s−1. The results indicate that the deformation behaviour of 2205/AH36 BC is similar to that of AH36 low carbon steel at high temperatures, and the softening mechanism of bimetal composite significantly depends on the imposed working temperatures and strain rates, which has been verified by the electron backscatter diffraction (EBSD) observation of microstructure evolution. Extra geometrically necessary dislocations (GNDs) were observed to accumulate in AH36 carbon steel layer adjacent to the interface after the hot working test at relatively low strain rates, resulting from the different thermal expansions and hot ductility between the 2205 stainless steel and AH36 carbon steel, and some existing defects on the interface from the previous hot rolling process. These observations can be employed as a reference to make the medium-thick bimetal composite be fabricated to the final products with hot working process in practice.