This paper carries out the structural topology optimization to minimize the radiated acoustic power in a thermal environment for the first time. The stress induced by the thermal environment can reduce the stiffness of the structure, thus changing the radiation property and optimal design. An approach to investigate this effect is presented through studying a baffled bi-material plate. The plate is excited by a harmonic load and subjected to a uniform temperature rise. The thermal stress is first evaluated and considered as pre-stress in the structural dynamic analysis. With the dynamic response, the acoustic power can be obtained using Rayleigh integral. Sensitivity analysis with respect to the design variables is calculated according to the material interpolation model. Some typical cases are studied; the thermal environment is below the critical buckling temperature and the driving frequency is lower than the plate's second natural frequency. Numerical results show that the natural frequencies decrease with the increase of the temperature and the structure tends to resonance; thus the radiated sound power level becomes higher and the pattern of the optimal topology resembles that of the associated mode shape more closely. The sound power level of the optimal plate becomes lower than that of the initial plate, especially for the higher temperature cases. During the optimization process, the critical buckling temperature increases and the structure is always in pre-buckling.