Interfaces and mechanical properties of the NM400/Q345 composite plates fabricated under different reduction ratios were investigated. An increasing in the cumulative reduction ratio enhanced the bonding interface of the NM400/Q345 composite plate by reducing the number of combinations with interfacial defects such as pores. The oxidation film at the interface was crushed and diffused after three passes of rolling (cumulative reduction ratio = 70%). The strength of the composite also increased with the increasing of cumulative reduction ratio. At the maximum cumulative reduction ratio (70%), the inter-diffusion of the matrix and composite metal was enhanced, and the microstructure became considerably refined. Furthermore, the elongation of the composite plate was decreased, which can be attributed to the reduced work hardening of the composite plate.