The study investigated the association between C-reactive protein-albumin ratio (CAR) and unplanned readmissions in 412 ulcerative colitis (UC) patients hospitalized at the Affiliated Hospital of Shandong University of Traditional Chinese Medicine. During the 1-year follow-up, the unplanned readmission rate was 27.43%. Each 1-unit increase in CAR was associated with a 126.9% higher risk of readmission after adjustment for confounding factors. Kaplan-Meier curves showed significant differences in readmission according to CAR, CRP, and albumin quartiles (log-rank test, P < 0.001). Restricted Cubic Splines revealed a positive linear correlation (P < 0.001) and a nonlinear relationship between CAR and readmissions. Subgroup analysis suggested an interaction between hypertension and CAR. CAR had good predictive value with area under the curve of 0.813 (1 month), 0.779 (3 months), 0.778 (6 months) and 0.799 (1 year).