The high reliability of the dual-active-bridge (DAB) converter is becoming a crucial requirement in dc grids. However, power device failure caused by temperature is one of the main factors restricting the reliability of the power electronics converter. Therefore, a thermal stress calculation method is proposed to determine the heat distribution inside the power modules employed in the DAB converter. First, considering the varying voltage conversion ratios and power loads, numerical loss models of active switches and their anti-parallel diodes are established in different soft-switching regions. Then, based on the 3-D finite element simulation, the junction-to-case thermal resistance is extracted to establish the thermal network of power modules. Finally, combined with the proposed loss and thermal models, the thermal stress and its distribution inside the power module under different voltage conversion ratios and loads are investigated.