In the pursuit of durable lithium–sulfur (Li–S) batteries, considerable research has been devoted to extending cycle life by developing advanced cathode materials. However, there exists a common overcharging problem which leads to the disfunction of Li–S batteries but has been largely overlooked. This study systematically investigates the overcharging failure and its underlying mechanisms. Experimental results reveal that failure comes from a soft internal short circuit (ISCs) caused by excessive lithium dendrite growth, primarily driven by the sulfur cathode rather than the lithium anode. Electrochemical processes during overcharging are examined, revealing the generation of a specific by-product. By analyzing the structural properties of the sulfur cathode such as topography and pore connectivity, and through combined experiments and theoretical simulations, the complex mechanisms through which the cathode influences lithium dendrite growth are elucidated. Finally, an effective "interlayer" strategy is proposed to mitigate the overcharging failure. This work sheds light on the overcharging mechanisms and emphasizes the critical importance of cathode design in improving the safety and reliability of Li–S batteries.