In order to obtain a good strength-plastic/toughness match relationship, 18Mn/40Si2CrMo multilayer composite steels were successfully fabricated by a vacuum hot rolling and warm rolling process in this paper. The effects of different warm rolling temperatures (400–600 °C) on the microstructure and mechanical properties of the multilayer composite steel were systematically investigated. The result shows that the warm rolling process reduces thickness of the interfacial diffusion layer, which improves the interfacial bonding strength of multilayer composite steel. With the increase of warm rolling temperature, the total elongation (TEL) increases but ultimate tensile strength (UTS) decreases. The multilayer composite steel with a warm temperature of 500 °C achieves the balance of strength and plastic of which the UTS and TEL are 1.7 GPa and 12.5%, respectively. This is due to the high work-hardening ability of deformation twins of the 18Mn layer and the precipitates nanoscale carbides of the 40Si2CrMo layer to obscure the dislocation movement.