Objective: to explore a better tracing of central lymph nodes in papillary thyroid cancer patients by comparing the differences of number and staining of central lymph nodes and nano-carbon extravasation between preoperative ultrasound-guided injection and intraoperative injection of nano-carbon. Methods: 227 patients were randomly enrolled into preoperative ultrasound guided injection group and intraoperative injection group. The number and staining of the lymph nodes in each sub-area of central region (linea alba cervical lymph nodes, prelaryngeal lymph nodes, pretracheal and paratracheal lymph nodes) were recorded and analyzed. Results: Central lymph nodes were found to be more and more accessible to be stained by preoperative injection of nano-carbon (p value＜0.05). Conclusion: Preoperative ultrasound guided injection of nano-carbon before surgery can reduce dye spillover and improve the staining rate and detection rate of central lymph nodes.