This paper presents an order reduction for the thermal dynamics of a diesel oxidation catalyst (DOC) with hydrocarbon (HC) dosing. The original model includes the pyrolysis of diesel droplets and a wall storage process in the upstream of the DOC. The order reduction process is derived from the thermodynamics model of the DOC for further control design. The results are compared with experimental data. It is found that the DOC can be simplified as a second-order model using the HC dosing model, which has more than 94% fitness, reflecting the thermodynamics of the system. According to this research, the DOC thermal dynamics can be considered to be equivalent to a time-varying second-order system for the investigation. The second-order parameters of K, Tw, and ζ are also investigated in this paper.