Abstract: The incidence and mortality of malignant tumors remain high, and difficulties in early diagnosis represent a major challenge in clinical diagnosis and treatment. Thymidine kinase 1(TK1) is an enzyme closely associated with cell proliferation and is specifically expressed during the cell cycle. Therefore, it serves as an ideal tumor biomarker. TK1 is upregulated in various malignant tumors and has important application value in early screening, dynamic disease monitoring, prognostic evaluation, and monitoring of treatment response in malignant tumors. In particular, when used in combination with conventional tumor markers, it can effectively improve diagnostic performance. In recent years, the potential of TK1 as a novel therapeutic target has also attracted considerable attention from researchers. This review therefore summarizes the biological characteristics of TK1 and research progress regarding its role in disease assessment in breast cancer, gynecological cancers, prostate cancer, lung cancer, gastrointestinal cancers, and liver cancer, aiming to provide a reliable reference for the rational application of TK1 in clinical practice for malignant tumors.