AIM: To evaluate refraction and ocular biological characteristics in children with unilateral congenital ptosis. METHODS: In this cross-sectional study, 200 Chinese children (3-15y) with unilateral congenital ptosis were evaluated. Cycloplegic refraction and ocular biological measurements were taken from Oct. 2020 to Aug. 2022. RESULTS: In patients with congenital ptosis, the prevalence of with-the-rule astigmatism and oblique astigmatism was significantly high. The cornea was flatter in ptotic eyes [K1 (42.37±1.62 vs 42.78±1.51 D), K2 (43.8±1.86 vs 44.2±1.64 D), corneal power (Km; 43.09±1.68 vs 43.49±1.53 D), all P<0.001]. Axial length (AL) was longer in ptotic eyes (22.55 vs 22.51 mm, P=0.012). The white-to-white (WTW) was significantly smaller in ptotic eyes (11.49 vs 11.65 mm, P<0.001). The central corneal thickness (CCT) was greater in ptotic eyes (553.50 vs 545.00 µm, P<0.001). No significant differences were found in anterior chamber depth (AD), lens thickness (LT) and vitreous thickness (VT) between ptotic and fellow eyes (P>0.05). In addition, the incidence of amblyopia in ptosis eyes was 32.0%, which was significantly higher than that in the normal population. CONCLUSION: Ptotic eyes have longer AL, flatter, thicker and smaller corneas than fellow eyes. The congenital ptosis increases the risk of amblyopia. The results suggest that regular examinations of refractive status and ocular biological parameters such as AL, are essential for children with unilateral congenital ptosis.