Objective To investigate the changes of residual radioactivity at different time points after 131I treatment in the patients with differentiated thyroid cancer(DTC)and influencing factors.Methods A to-tal of 235 patients with DTC receiving 131I treatment in this hospital from January 2021 to June 2023 were se-lected as the study subjects and divided into the high dose group(＞5.55 GBp,n=56)and low dose group(≤5.55 GBp,n=179)according to the treatment dose.The clinical data of the two groups were collected and the changes of residual radioactivity after 131I treatment were compared between the two groups.The binary re-gression was used to analyze its influencing factors.Results The sex,age,BMI,basic metabolic rate(BMR)and serum thyroglobulin antibody(TgAb)had no statistical differences between the two groups(P＞0.05).The proportions of serum thyroglobulin(TG)＜1 ng/mL,131I first time treatment and residual thyroid ratio prompted by the whole body 131I scan after treatment in the low dose group were significantly higher than those in the high dose group(P＜0.05).The residual radioactivity in the two groups was significantly de-creased with time extension.The residual radioactivity at 24,48,72 h after treatment in the low dose group was significantly lower than that in the high dose group(P＜0.05).The binary logistic regression analysis re-sults showed that the T stage and treatment dose were the influencing factors of residual radioactivity after 131I treatment.Conclusion The residual radioactivity after 131I treatment in the patients with DTC shows the significant decreasing trend with time extension,this change trend has an active significance for further opti-mizing and perfecting the isolation and protection scheme.For the patients with high T stage and big treat-ment dose,the isolation time should exceed 72 h.