Purpose To investigate the expression of SMOC2 in papillary thyroid carcinomas(PTC)and its efficacy in joint diagnosis with CK19,Galectin-3,MC and BRAF V600E.Methods Bioinformatics was uesd to analyze the mR-NA expression differences of SMOC2 in PTC and benign thyroid tissues in the Gene Expression Omnibus database and The Canc-er Genome Atlas database.Detection of SMOC2 protein expres-sion in paraffin tissue of 75 cases of PTC and 45 cases of papilla-ry thyroid hyperplasia(PTH)was used by using EnVision meth-od,combined with CK19,Galectin-3,and MC and BRAF V600E for sensitivity and specificity analysis.Results The bioinformatics analysis results showed that the mRNA expression level of SMOC2 in PTC tissue was significantly lower than that in benign thyroid tissue(P<0.05),and the area under the curve(AUC)predicted by SMOC2 for PTC diagnosis was 0.910(P<0.001).The immunohistochemical results showed that the ex-pression of SMOC2 in PTC was significantly lower than that in PTH tissue(P<0.001),and the AUC of SMOC2 for PTC diag-nosis was 0.898(P<0.001).The AUC of SMOC2 combined with CK19,Galectin-3,MC and BRAF V600E in the diagnosis of PTC was 1.000(P<0.001),and the AUC values of the combination of other markers were lower than 1.000.Conclu-sion The expression of SMOC2 in PTC is significantly de-creased,which can be used as an important marker for the diag-nosis and differential diagnosis of PTC.Combined with CK19,Galectin-3,MC and BRAF V600E,the sensitivity and specifici-ty of PTC can be improved to a certain extent.