OBJECTIVE To compare the efficacy and safety of vedolizumab （VDZ） versus infliximab （IFX） in biologic-naive（Bio-naive） patients with moderate-to-severe active ulcerative colitis （UC）， and to analyze the factors influencing efficacy. METHODS Clinical data were retrospectively collected from Bio-naive patients with moderate-to-severe active UC who received treatment in the Department of Gastroenterology at Shanxi Provincial People’s Hospital from June 2023 to June 2024. Based on the type of biologic agent administered， the patients were divided into the IFX group （41 cases） and the VDZ group （30 cases）. Patients in the two groups received IFX （5 mg/kg） or VDZ （300 mg） for induction and maintenance of remission therapy. The two groups were compared regarding modified Mayo score， serological indicators （hemoglobin， albumin， platelet count， erythrocyte sedimentation rate， C-reactive protein）， combined medication， efficacy-related indexes （clinical response rate/remission rate， and endoscopic response rate/remission rate）， and the occurrence of adverse drug reactions （ADR）. Based on Logistic regression model， univariate and multivariate analyses were conducted to identify potential factors influencing clinical remission at week 14 and endoscopic remission at week 38. RESULTS There were no statistically significant differences in clinical response rate/remission rate， or endoscopic response rate/remission rate between the two groups at weeks 14 and 38 （ P ＞0.05）. However， at week 14 of treatment， the proportion of patients using concomitant corticosteroids in VDZ group was 26.67%， significantly higher than the 7.50% in IFX group （ P ＜0.05）. There was no statistical significance in the overall incidence of ADR between the two groups （ P ＞0.05）； all ADRs in the IFX group were grade 3 and led to treatment discontinuation （6 cases）， whereas ADR in the VDZ group was grade 2 and did not interrupt therapy （1 case）. Univariate and multivariate regression analyses revealed that disease type （relapsing） was significantly associated with clinical remission at week 14 of treatment， and a history of smoking was significantly associated with endoscopic remission at week 38 of treatment （the odds ratios were 0.08 for both， with 95% confidence intervals of 0.01-0.77 and 0.01-0.91 respectively， P ＜0.05）. CONCLUSIONS For Bio-naive patients with moderate-to-severe active UC， VDZ and IFX demonstrate comparable efficacy in inducing and maintaining clinical remission and promoting mucosal healing， as well as overall safety. Although IFX can achieve faster control of inflammation in the early stage of the disease， it causes more severe ADR. Disease type （relapsing） and smoking history are identified as independent negative predictors for short-term clinical remission and long-term endoscopic remission， respectively.