The study investigated the relationship between ocular axial length (AL) and anterior segment biometry in Chinese students aged 7 to 18 years with high axial myopia. It included 366,278 participants from January 2020 to December 2022, of whom 28,877 had high axial myopia (AL ≥ 26 mm). Mean keratometry (K) and lens thickness (LT) were significantly lower in this group than in the non-myopic and normal myopic groups (all p < 0.001). Conversely, central corneal thickness (CCT) and anterior chamber depth (ACD) were significantly greater (all p < 0.001). Multivariate linear regression showed a correlation of K (β = −0.265), CCT (β = 0.001), ACD (β = 1.609) and LT (β = −0.607) with AL (all p < 0.001). For AL < 27 mm, AL was negatively correlated with LT, while for AL > 27 mm there was no linear correlation (p > 0.05). High AL (≥ 26 mm) was associated with lower K, thicker CCT, deeper ACD and thinner LT compared to AL < 26 mm. At AL < 27 mm the thinning of the LT compensates for myopia, at AL > 27 mm this effect disappears.