Flexible strain‐sensitive‐material‐based sensors are desired owing to their widespread applications in intelligent robots, health monitoring, human motion detection, and other fields. High electrical–mechanical coupling behaviors of 2D materials make them one of the most promising candidates for miniaturized, integrated, and high‐resolution strain sensors, motivating to explore the influence of strain‐induced band‐gap changes on electrical properties of more materials and assess their potential application in strain sensors. Herein, a ternary SnSSe alloy nanosheet‐based strain sensor is reported showing an enhanced gauge factor (GF) up to 69.7 and a good reproducibility and linearity within strain of 0.9%. Such sensor holds high‐sensitive features under low strain, and demonstrates an improved sensitivity with a decrease in the membrane thickness. The high sensitivity is attributed to widening band gap and density of states reduction induced by strain, as verified by theoretical model and first‐principles calculations. These findings show that a sensor with adjustable strain sensitivity might be realized by simply changing the elemental constituents of 2D alloying materials.