A multi-unit thermoradiative device (TRD) is used for automotive exhaust waste heat recovery in this study. 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 gas 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 the increase of unit number and also decreases with the increase of 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 current rising, reflecting the coupling between electrical output and thermal processes. Increasing the 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. Because of the nonlinear relationship between electrical power and heat flux, efficiency attains an optimum value at a specific voltage, achieving a balance between electrical output and heat dissipation. This study demonstrates that the locally optimal power reaches a global maximum value of 170.45 W at a bandgap of 0.06 eV, whereas the locally optimal efficiency increases monotonically with the increase of 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 study provides a new method for optimizing the performance of thermoelectric systems.