Objective: To evaluate the effect of ultraviolet A/riboflavin corneal crosslinking (CXL) on keratoconic  patients with a corneal thickness less than 400 μm. Methods: In this prospective study, patients with  keratoconus were divided into a thin cornea group with a corneal thickness (CT) less than 400 μm and a  thick cornea group with a CT more than 400 μm. The patients in the thin and thick cornea groups were  treated with a hypotonic or isotonic riboflavin solution before and during CXL. Corneal morphological  parameters were evaluated before and after CXL during a 3-year follow-up. The data were analyzed with a  repeated measures ANOVA, t test, Wilcoxon rank sum test or Mann-Whitney U test. Results: After surgery,  the maximum keratometry (Kmax) of the thin and thick cornea groups underwent a continuous decrease  (F= 24.364, P<0.001; F=10.427, P=0.001). In the thin group, the Kmax value was 60.51±6.11 diopters  (D) before surgery and significantly decreased to 57.43±6.82 D, 56.13±6.85 D and 54.97±6.66 D at 1, 2  and 3 years after surgery (t=3.670, P=0.002; t=4.637, P<0.001; t=5.816, P<0.001). In the thick group, the  Kmax value was 54.56±6.27 D before surgery and significantly decreased to 53.25±6.42 D, 52.32±6.47  D and 51.58±6.70 D at 1, 2 and 3 years after surgery (t=2.266, P=0.040; t=3.302, P=0.005; t=3.769,  P=0.002). The Kmax value of the thin cornea group before surgery was higher than that of the thick  cornea group (t=2.714, P=0.011). There were no significant differences in Kmax between the two groups  at 1 year, 2 years or 3 years. In the thin group, there were significant differences between the preoperative  and 3-year postoperative visual acuity (UCVA), best corrected visual acuity (BCVA) and thinnest corneal  thickness (TCT) (Z=-2.716, P=0.007; Z=-3.063, P=0.002; t=4.468, P<0.001). In the thick group, there  were significant differences between the preoperative and 3-year postoperative UCVA, BCVA and TCT  (t=3.572, P=0.003; Z=-2.956, P=0.003; Z=-3.410, P=0.001). In the two groups, there were no significant  differences between the preoperative and 3-year postoperative intraocular pressure (IOP) or endothelial cell  density (ECD). There were no significant differences in pre- or postoperative UCVA, BCVA, IOP, or ECD  between the two groups. There were significiant differences in pre- and postoperative TCT between the two  groups (Z=-4.816, P=0.001; Z=-4.024, P<0.001). Conclusion: CXL is an effective and safe way to halt  disease progression and improve visual acuity in keratoconic patients with a CT less than 400 μm.