Purpose To explore the application value of ultrasound-guided thyroid fine needle aspiration liquid-based thin layer cytopathology combined with p21 and Cyclin D1 detection in preoperative diagnosis of papillary thyroid carcinoma.Meth-ods Immunocytochemical staining was used to detect the ex-pression differences of p21 and Cyclin D1 between benign and malignant thyroid nodules,and their correlations the clinicopath-ological features.The diagnostic efficacy of US-FNAB,p21,Cyclin D1 and the three combined detection in benign and malig-nant thyroid nodules was evaluated by constructing receiver oper-ating curve.Results The expression of p21 and Cyclin D1 was up-regulated in the papillary thyroid carcinoma group,86.36%(57/66)and 93.94%(62/66),and 1.96%(1/52)and 5.77%(3/52)in the benign nodule group,respectively;the difference between the two groups was significant(P<0.05).The positive rates of p21 and Cyclin D1 in BRAF V600E wild type PTC were 88.89%(8/9).The expression of p21 and Cyc-lin D1 was correlated with the tumor size of PTC(P<0.05),but not with gender,age,number of tumor foci,lymph node metastasis,and TNM stage(P>0.05).The sensitivity,speci-ficity,positive predictive value and negative predictive value of US-FNAB,p21 and Cyclin D1 combined detection were 95.45%,98.07%,98.43%and 94.44%,respectively,which were higher than those of US-FNAB independent detection,and the sensitivity and negative predictive value were higher than those of BRAF V600E.The area under ROC curve of US-FNAB,p21 and Cyclin D1 combined detection(AUC = 0.967 7)was larger than that of US-FNAB(AUC = 0.849 9)and the difference between the two was statistically significant(P<0.05).The area under ROC curve of the combined detection of the three was greater than that of independent detection of BRAF V600E(AUC =0.931 8),and the combined detection of US-FNAB with p21(AUC = 0.946 4)or Cyclin D1(AUC = 0.944 3).It was close to that of US-FNAB combined BRAF V600E detection(AUC = 0.971 2).Conclusion US-FNAB combined with p21 and Cyclin D1 immunohistochemical detec-tion can help improve the sensitivity of preoperative diagnosis of papillary thyroid carcinoma,and it has high diagnostic value for BRAF V600E wild-type papillary carcinoma.