Objectives To examine the ocular biometric parameters and predict the annual growth rate of the physiological axial length (AL) in Chinese preschool children aged 4-6 years old.Methods This retrospective cross-sectional study included 1090 kindergarten students (1090 right eyes) between the ages of 4 and 6 years from Pinggu and Chaoyang District, Beijing. Dioptre values were ascertained following cycloplegic autorefraction. Predicted AL was obtained through the application of the Gaussian process regression model as an optimisation technique. Subsequently, the annual growth rate of physiological AL for non-myopic preschool children (n=1061) was computed via the backward difference method.Results In total, 85.4% of preschool children (931 individuals) had hyperopic refractive status in the 4-6 years age group, while only 2.7% (29 individuals) showed myopia. Boys had longer AL, larger AL-to-corneal radius ratio, deeper anterior chamber depth and lower lens power. The average physiological axial growth for boys and girls ranged from 0.050 mm/year to 0.165 mm/year and 0.063 mm/year to 0.168 mm/year, respectively. As age increased, the physiological AL growth in non-myopic cases decreased. Additionally, as hyperopic spherical equivalent refraction error lessened, the physiological AL growth component slowed down.Conclusions In preschool children, refractive development predominantly exhibits mild hyperopia. The concept of physiological AL provides valuable insights into the complexities of ocular development.