Background: To identify the capsule enlargement index after femtosecond laser-assisted anterior capsulorhexis in 2–6-year-old children who underwent congenital cataract surgery. Methods: In this prospective case series study, femtosecond laser-assisted anterior capsulorhexis was performed in patients with congenital cataract, aged 2–6 years. The actual achieved capsulorhexis diameters were measured with Digimizer version 4.2.6. Correlation coefficient (r) and multiple linear regression analysis were used to evaluate the variables that could potentially influence anterior capsulorhexis enlargement index (E). Results: This prospective study enrolled 28 eyes of 22 patients with congenital cataract. The mean age of the patients at surgery was 4.67 years ± 1.54 (standard deviation [SD]). “E” of the 28 cases was 1.211±0.039 (SD). Correlation analysis showed that “E” correlated significantly with the anterior chamber depth (ACD) (r=−0.469, p=0.021) and axial length (AL) (r=0.452, p=0.027). The following formula was developed by using multivariable linear regression analysis: Predicted E=1.177−0.052×ACD +0.009×AL, R2=0.346 (F=4.396, p=0.046).Conclusions: The anterior capsulorhexis enlargement index and its calculation formula could help to set up an accurate programmed capsulorhexis diameter for femtosecond laser-assisted congenital cataract surgery in children aged 2–6 years. Thus, an appropriate actual capsulorhexis diameter could be achieved