Abstract: Objective To investigate the expression of cathepsin L (CTSL) in papillary thyroid carcinoma (PTC), and analyze the relationship between the expression of CTSL and the clinicopathological parameters or the prognosis in patients. Methods Total 100 cases of PTC tumor tissue and non-tumble tissue were studied. Immunochemistry was used to detect the expression of CTSL protein in cancer tissues, and the relationship between PAGE4 protein and parameters or prognosis of patients was analyzed. Results The positive rate of CTSL expression in cancer tissues was 100.00%, which was significantly higher than that in adjacent normal tissues 5.00%, the difference was statistically significant (χ²=87.947, P<0.001). The high expression rate of CTSL was 50.00%, 80.00%, 100.00%, 100.00% in TNM stage Ⅰ, Ⅱ, Ⅲ, Ⅳ, respectively. There were significant difference among the four groups (χ²=16.875, P=0.001). The high expression rate of CTSL was 97.06% in lymph node metastasis, which was significantly higher than that without lymph node metastasis 57.69%, the difference was statistically significant (χ²=11.916, P=0.001). The high expression rate of CTSL in the primary tumor was 82.35%, which was significantly lower than that in the lymph node metastasis 97.06%, the difference was statistically significant (χ²=2.548, P=0.110). Conclusion The up-regulation of CTSL expression in PTC tumor tissue is closely related to the pathological parameters such as TNM stage and lymph node metastasis, and it can used as a predictor of disease progression and lymph node metastasis potential.