This study employs a multi-unit thermoradiative device (TRD) for automotive exhaust waste heat recovery. A coupled model integrating radiative heat transfer, current-voltage characteristics, and fluid heat exchange is established. Based on Fourier’s law of heat conduction and thermal radiative transfer theory, the energy constraint equations, total power output, and conversion efficiency of the system are derived. The variations of exhaust temperature, TRD operating temperature, and ambient temperature with unit number are obtained through numerical simulations, thereby revealing the regulation mechanisms of voltage and semiconductor bandgap on energy conversion performance. Results show that the temperatures of the exhaust gas and the hot side of the TRD decrease with increasing unit number and also decline with increasing current at the same unit position. In contrast, the cold side of the TRD and the ambient temperature rise due to heat accumulation and cascading heating effects, and further increase with higher current, reflecting the coupling between electrical output and thermal processes. Increased voltage suppresses radiative recombination, leading to reduced current, while the electrical power reaches a maximum at a specific operating point. The total heat flux is reduced as voltage increases. Due to the nonlinear relationship between electrical power and heat flux, the efficiency attains an optimum value at a certain voltage, achieving a balance between electrical output and heat dissipation. This study demonstrates that the locally optimal power reaches a global maximum of 170.45 W at a bandgap of 0.06 eV, while the locally optimal efficiency increases monotonically with bandgap before saturating gradually. To address the inherent trade-off between power and efficiency, a target function Z defined as the product of locally optimal power and efficiency is introduced. Numerical analysis reveals that Z attains its maximum value of 49.74 W at a bandgap of 0.105 eV, effectively balancing the competing objectives of power output and energy conversion efficiency. This approach offers a new pathway for performance optimization in thermoelectric systems.