Objective:To investigate the clinical outcome after surgery and first 131I treatment in patients with moderate-risk papillary thyroid cancer (PTC), and analyze the relevant factors that affect the therapeutic effect. Methods:From January 2018 to April 2019, 135 patients (48 males, 87 females; age (42.7±11.1) years) with moderate-risk PTC in the Second Affiliated Hospital of Chongqing Medical University were retrospectively analyzed. According to the 2015 American Thyroid Association (ATA) guidelines, patients were divided into excellent response (ER) group, inderteriminate response (IDR) group, biochemical incomplete response (BIR) group and structural incomplete response (SIR) group, of which IDR, BIR, SIR were collectively referred to as the non-ER group. χ2 test and Mann-Whitney U test were used to compare the general clinical features between the ER and non-ER groups, and then multivariate logistic regression analysis was performed. The predicted value of pre-ablation stimulated thyroglobulin (ps-Tg) to ER was assessed by ROC curve analysis. Results:The treatment responses of 94 patients were ER, and those of 41 were non-ER. The differences in tumor size (0.80(0.50, 1.10) vs 1.00(0.55, 1.50) cm; U=1 491.50, P=0.036), the number of metastatic lymph nodes (3(2, 5) vs 4(2, 12); U=1 422.00, P=0.015), metastatic lymph node size (0.50(0.30, 0.65) vs 0.50(0.30, 1.45) cm; U=1 396.50, P=0.013), metastatic lymph node involvement rate (50%(30%, 70%) vs 60%(50%, 85%); U=1 441.50, P=0.024), metastatic lymph node location (central/lateral: 76/18 vs 24/17; χ2=7.40, P=0.007) and ps-Tg level (2.1(0.8, 5.3) vs 14.0(3.2, 35.2) μg/L; U=680.00, P<0.001) were statistically significant between the ER and non-ER groups. Multivariate logistic regression analysis showed that ps-Tg (odds ratio ( OR)=1.200, 95% CI: 1.107-1.302, P<0.001) was an independent factor influencing ER. The cut-off value of ps-Tg for predicting ER was 7.38 μg/L, with the sensitivity and specificity of 68.3%(28/41) and 87.2%(82/94) respectively. Conclusion:Moderate-risk PTC patients with smaller tumor size, fewer metastatic lymph nodes, lower metastatic lymph node involvement rate, metastatic lymph nodes in central area, smaller metastatic lymph node size, and ps-Tg<7.38 μg/L have better therapeutic effect after initial 131I treatment.