Thyroid cancer (TC) is the most common endocrine tumor. In recent years, with the population-based disease screening programs worldwide and the lifestyle changes, the incidence rate of thyroid cancer keeps rising. At present, the monitoring of diagnosis and treatment results of thyroid cancer mainly depends on invasive tissue biopsy or repeated imaging procedures with radiation exposure. Although the prognosis of most thyroid cancer patients is favorable, the recurrence and mortality rates are relatively high when extensive metastasis occurs. Therefore, developing a non-invasive, simple and repeatable detection technique in the early diagnosis and recurrence monitoring of thyroid cancer is particularly important. Liquid biopsy is a non-invasive testing procedure that can extract circulating tumor cells (CTC), circulating free DNA (cfDNA), microRNAs (miRNAs) and other biological samples from patients' blood, urine and other body fluids for analysis to obtain valuable biological information for tumor diagnosis and treatment, by which the clinicians can repeatedly and dynamically analyze the development of the patients'tumor, so as to achieve the purpose of accurate medical treatment. Among them, the cfDNA detection technique is the most important. It is a kind of acellular extracellular DNA existing in the body fluids of animals, plants and humans. Through the quantitative analysis, integrity detection, specific gene mutation and gene methylation analysis of cfDNA in blood, it plays a role in the early diagnosis, prediction of metastasis, recurrence and treatment effect monitoring of malignant tumors. Because of its non-invasive, high specificity and sensitivity, it has gradually become a hot spot in the research of cancer diagnosis and treatment, and its mechanism is still not fully understood so far. In recent years, the relevant literature evaluating the role of cfDNA detection in thyroid cancer has increased, which further shows that cfDNA detection technique can play an important role in the diagnosis, treatment and disease monitoring of thyroid cancer, and become a valuable tool to overcome the limitations of routine examination and monitoring methods of thyroid cancer. Therefore, cfDNA may become a new molecular marker to judge the effect of diagnosis and treatment of thyroid cancer. Here, the authors review the latest research progress of cfDNA in the diagnosis and treatment of thyroid cancer.