Real-time detection of lithium plating under extreme conditions is critical for battery safety. Here, we use embedded fiber Bragg grating (FBG) sensors to achieve real-time monitoring of mechanical strain evolution in operating graphite pouch cells. At ultralow temperatures (−20 and −30 °C), we observe an anomalous suppression in differential strain (dε/dQ) during late-stage charging. Specifically, the strain generation rate is severely constrained, dropping to approximately 0.1 με·mAh⁻¹ (με = 10⁻⁶ strain) at high state-of-charge (SOC)—a stark contrast to the normal intercalation baseline of ~ 0.3 με·mAh⁻¹ observed under milder, non-plating conditions. Through synchronized electrochemical analysis and post-mortem characterization, we show that this mechanical anomaly reflects a fundamental shift in plating behavior: At cryogenic temperatures, internal mechanical constraints imposed by prior plating and solid electrolyte interphase (SEI) densification suppress further expansion, altering the progression of lithium deposition. The real-time strain signals also reveal a dynamic competition between SEI fracture–repair cycles and lithium re-plating, providing insight into coupled degradation mechanisms governing battery failure. This work establishes an in-situ diagnostic tool for battery failure and offers new mechanistic understanding of lithium plating under extreme conditions.