Lithium-sulfur (Li-S) batteries are competitive for next-generation energy storage applications. However, soluble polysulfides cause severe shuttle effect and electrolyte abuse. Solid-solid and quasi-solid conversions are effective to address the problems, but the mechanistic understanding is still incomplete. In particular, the internal stress evolution which is tied closely to the electrode structure evolution, is rarely explored due to lack of appropriate characterization methods. Here, we report in situ monitoring of cathode stress evolution in Li-S batteries via optical fiber Bragg grating (FBG) sensors. Stress evolution in three classic mechanisms of Li-S cells including solid-liquid-solid, solid-solid and quasi-solid mechanisms has been systematically investigated. The results indicate that the differences in stress evolution are closely related to the evolution characteristics of cathode structure and volume changes. The proposed method offers a unique view for understanding the chemo-mechanics in Li-S cells, which can be extended to different electrode materials.