Objective: To investigate the ocular biological characteristics of occult high myopia (OHM) in children and to provide objective evidence for early screening of the disease. Methods: A cross-sectional study was conducted. A total of 235 myopic children (470 eyes) aged 6–12 years who visited the ophthalmology clinic of Yanting County People''s Hospital from September 2024 to September 2025 were enrolled. The participants were divided into a control group (common myopia, 120 cases, 240 eyes) and an observation group (OHM, 115 cases, 230 eyes). Axial length (AL), mean keratometry (Km), spherical equivalent (SE), axial length/corneal radius of curvature ratio (AL/CR), axial length/mean keratometry ratio (AL/Km), subfoveal choroidal thickness (SFCT) and retinal thickness (RT) were measured and compared between the two groups. The diagnostic performance of each parameter was evaluated using receiver operating characteristic (ROC) curves. Results: The control group consisted of 63 males and 57 females, with a mean age of 8.60 ± 1.10 years, while the observation group consisted of 50 males and 65 females, with a mean age of 8.63 ± 1.09 years. There were no statistically significant differences in age or sex between the two groups (P > 0.05). Compared with the control group, the observation group showed significantly higher AL [25.69 (25.27, 26.09) mm vs. 23.77 (23.64, 23.91) mm] and AL/Km [0.636 (0.625, 0.651) vs. 0.543 (0.536, 0.55)], and significantly lower Km [40.5 (40, 40.5) D vs. 43.5 (43.5, 44.5) D] (all P<0.001). No significant difference in AL/CR was found between the two groups (P=0.106). The observation group also exhibited significantly thinner foveal RT, SFCT and inner nasal RT compared with the control group (all P<0.05). ROC analysis showed that AL/Km achieved an area under the curve (AUC) of 0.998 (95% CI: 0.996–1.000) for identifying OHM, with an optimal cutoff value of 0.583. AL and Km both showed an AUC of 1.000, which was attributed to deterministic bias caused by the inclusion criteria, and thus their diagnostic performance cannot be extrapolated to clinical practice. AL/CR had no diagnostic value (AUC<0.5). Conclusion: Children with OHM are characterized by long axial length, flat cornea and relatively low myopic refraction despite excessive axial elongation. Structural damage to the macular retina and choroid is already present. The axial length/mean keratometry ratio (AL/Km) shows good diagnostic performance with a cutoff value of 0.583, making it a promising indicator for early screening of OHM in children.