Battery safety has emerged as an increasingly critical issue. During the failure process of lithium-ion batteries, the release of hydrogen gas is one of the characteristic byproducts of thermal runaway in lithium batteries. A hydrogen sensor with fast responsiveness, high sensitivity, accuracy, and reliability for real-time monitoring of environmental hydrogen concentration plays a pivotal role in assessing the health status of lithium-ion batteries and enabling early warning of thermal runaway. In this study, SnO2 nanosheet materials were synthesized via a hydrothermal method, and their hydrogen-sensing performance was systematically investigated. The results demonstrate that the SnO2 nanosheet gas sensor exhibits a response value of 11.64 to 1000 ppm hydrogen gas, with response time of 22 s and recovery time of 18 s. Within the hydrogen concentration range of 50–2000 ppm, the sensor shows a strong linear correlation between the response value and gas concentration. Repeatability experiments confirm that the SnO2 nanosheet gas sensor features high sensitivity, excellent reproducibility, and stable performance toward hydrogen gas.