Abstract: Large-capacity square lithium iron phosphate batteries serve as core energy storage carriers for new energy storage and grid frequency regulation scenarios. The strain evolution characteristics on the shell surface and internal multi-scale coupling mechanisms during long-term charge-discharge cycles have not been systematically clarified. Uneven expansion and continuous stress accumulation in cycling may easily trigger electrode pulverization,shell bulging and even thermal runaway risks, which restricts the structural optimization, health monitoring and long-life design of large- capacity energy storage batteries. Distributed fiber Bragg grating sensing technology is adopted in this work. A commercial 280 Ah square aluminum-shell lithium iron phosphate battery is selected as the test sample. Three groups of bare fiber strain measuring points and sleeve-packaged temperature compensation measuring points are arranged on the front and side surfaces of the battery under a constant temperature environment of 25±2 ℃. A charge-discharge cycling system and synchronous demodulation platform for fiber gratings are estab-lished to carry out complete charge-discharge cycling tests at 0.5 C rate. Coupled time-series data of strain, tempera-ture and voltage at multiple positions are collected synchronously. The spatial distribution law of strain in a single cycle and staged evolution characteristics of strain during long-term cycling are systematically analyzed, and the inherent mechanism of mechanical anisotropy of batteries under the coupling effect of shell constraint and winding core expansion is illustrated. Test results indicate that the surface strain of the battery changes periodically in tension and compression with the state of charge during a single charge-discharge cycle. Multi-stage lithium intercalation phase transformation of graphite anodes leads to obvious growth platforms of strain curves at 30% SOC and 100% SOC. Obvious spatial heterogeneity and anisotropy exist in battery strain. The peak strain at the geometric center of the front surface is about 420 με, while that of the side surface reaches 480 με. The overall strain amplitude of the side surface is 27% higher than that of the front surface. The narrow shell has weak structural stiffness and is more sensitive to internal expansion deformation. The strain throughout long-term cycles follows a unified three-stage evolution rule. In the activation expansion stage, the strain rises continuously with a total increment of 50–100 με. In the rapid decline stage in the middle of cycling, the overall strain decreases by 150–200 με. In the stable fluctua-tion stage at the later cycling period, the strain fluctuates within ±50 με. Distinct evolution differences are observed among measuring points. The diagonal measuring points on the front surface fully present the three-stage variation trend, while the central area maintains high tensile strain without an obvious decline. The geometric center of the side surface transforms from tensile strain to compressive strain of approximately ?240 με in the later cycling stage, and the strain at diagonal measuring points approaches 0 με, which acts as the force balance point of shell constraints. The research conclusions can provide important theoretical basis and technical support for mechanical performance evaluation, structural optimization design and health state monitoring of large-capacity energy storage batteries.