Abstract: The Solid Oxide Fuel Cell （SOFC） is a highly efficient power generation device that directly converts the chemical energy in fuel into electrical energy through high-temperature electrochemical reactions, boasting exceptionally high theoretical conversion efficiency. However, its internal operating temperature is high （exceeding 700 °C） and has a large temperature gradient, which results in significant thermal stress on various structural components during operation. This, in turn, causes deformation of the internal micro and macro structures, leading to a decline in electrochemical catalytic performance and, in severe cases, can result in damage to the fuel cell stack structure. Improving the thermal stress distribution within the stack can increase stack reliability, reduce the risk of structural damage, and extend the lifespan of the stack. A three-dimensional multi-physics coupled numerical model for a 5-layer planar fuel cell stack was established. While maintaining the same effective reaction area of the cathode, a series of thermal stress changes in the stack were analyzed when applying single cells with different length to width ratios. The results show that adjusting the length to width ratio of the single cells significantly affects the temperature distribution within the stack and the thermal stress distribution of each component. Increasing the length to width ratio of the single cells can improve the temperature distribution of the stack, effectively reduce thermal stress within the stack, and improve the stress distribution. When the length to width ratio is increased to 2.8∶1, compared to the most common 1∶1 square single cell, the highest temperature of the stack decreases from 1128 K to 1106 K, and the maximum temperature difference decreases from 107 K to 81 K. Besides the significant improvement in temperature distribution, the magnitude of stress distribution can be reduced by more than 40%. The maximum principal stress in the electrolyte decreases from 81.5 MPa to 46.8 MPa, and the maximum principal stress values in the anode, cathode, and sealant decrease from 46.3 MPa, 31.3 MPa, and 21.1 MPa to 21.1 MPa, 11.3 MPa, and 9.7 MPa, respectively. Therefore, reasonably increasing the length to width ratio of the single cells is an effective way to reduce thermal stress in the stack.