Liver injury, caused by factors like viral hepatitis and drug overdose, poses a significant health risk, with current diagnostic methods lacking specificity, increasing the need for more precise molecular imaging techniques. Herein, we present an activatable semiconducting liver injury reporter (SLIR) for early and accurate diagnosis of liver injury. The SLIR, which is composed of semiconducting polymers with an electronwithdrawing quenching segment, remains nonfluorescent until it encounters biothiols such as cysteine in the liver. SLIR accumulates efficiently in the liver and respond rapidly to biothiols, allowing accurate and early detection of liver damage. The recovery of SLIR fluorescence negatively reflects the dynamics of oxidative stress in the liver and provides information on the severity of tissue damage. Thus, the specificity of SLIR, the fast response, and the efficient targeting of the liver make it a promising tool for the precise diagnosis of liver damage at an early stage.