Orthokeratology (ortho-K) is an established intervention for myopia control in children, yet the vascular mechanisms within the choroidal sublayers underlying its efficacy remain unclear. This study aims to investigate the changes in the choroidal vascularity index (CVI) of choroid sublayers after ortho-K treatment, the association between 1-month CVI changes and 1-year axial length (AL) elongation, and the role of CVI responses in predicting the 1-year control efficacy of ortho-K treatment.A prospective cohort study was conducted, enrolling 257 children with myopia aged 7-16 years. The subjects wore ortho-K lenses continuously for 12 months. Choroidal volume, vascular volume (CVV), stromal volume (CSV), and CVI of the choroid sublayers were assessed using optical coherence tomography images at baseline and 1-month follow-up. The total choroidal layer (TCL) includes the large-vessel choroidal layer (LVCL) and the medium-vessel choroidal layer (MVCL). The Early Treatment of Diabetic Retinopathy Study (ETDRS) grid divides the macular area into nine subfields for detailed analysis.The 1-month CVI of TCL and MVCL in all ETDRS grid regions were significantly lower than the baseline CVI (P ≤ 0.036), except for the superior outer ring subfield in TCL (P = 0.070). After the 1-month ortho-K treatment, both choroidal volume and CSV significantly increased in all choroidal sublayers compared with baseline data (P ≤ 0.027), while CVV also showed significant increases in half of the ETDRS grid regions (14/27). After adjusting for age and gender, the baseline CVI of temporal inner ring subfield in TCL (β = 0.146, P = 0.006), and 1-month CVI changes of the temporal outer ring subfield (β = 0.337, P < 0.001) and nasal inner ring subfield (β = 0.174, P = 0.002) in MVCL, were independently associated with 1-year AL elongation during ortho-K treatment. On the basis of the 1-month CVI changes of temporal outer ring subfield and nasal inner ring subfield in MVCL, as well as age and sex, the prediction model demonstrated an area under the curve (AUC) of 0.905 (95% CI 0.870-0.941) for distinguishing children with myopia who exhibited slow versus fast AL elongation.After the 1-month ortho-K treatment, choroidal volume, CVV, and CSV, of all choroidal sublayers significantly increased, while CVI of TCL and MVCL significantly decreased. The CVI in LVCL showed no significant changes. The differences in short-term CVI changes among choroidal sublayers indicate disproportionate changes in CVV and CSV, which help to explore the possible choroidal mechanisms of myopia control with ortho-K treatment. Early CVI changes in MVCL possess potential clinical significance for predicting long-term AL elongation.© 2025. The Author(s).