Background <p>Myopia has been a major public health problem worldwide, with the highest incidence of myopia in East Asia. Investigating the incidence of myopia (non-cycloplegic) and its associated factors among children in primary and junior high school in underdeveloped regions of China.</p> Methods <p>This observational cohort study was completed in Shihezi, China, which was drawn from nine primary and secondary schools with a follow-up period from 2018 to 2023, all of which were randomly selected in proportion to the number of schools in the city. Students (with or without myopia (non-cycloplegic)) who have been able to participate continuously for three consecutive years starting from 2018 were eligible for inclusion. Data analysis occurred from April 2025 and May 2025.</p> Results <p>A total of 3,551 primary (median [P25, P75] age 7.5 [6.8, 9.0] years; 1077/2345 [45.9] girls) or junior high school (median [P25, P75] age 12.8 [12.4, 13.1]years; 585/1206 [45.9] girls) students were included. The annual incidence of myopia was 10% to 21% in primary and junior high schools. A higher hyperopic reserve in primary and junior high school students served as a protective factor against myopia onset. Female sex and parental myopia were risk factors for myopia incidence in primary school students, while &gt; 8&#xa0;h of sleep was protective in this group. Furthermore, the hyperopic reserve progressively disappeared from grades 1–2. Based on the SHAP model, the order of importance of the factors influencing the incidence of myopia was SE, segment, weight, sleep time, height, gender, and parental myopia.</p> Conclusions <p>The significantly higher prevalence of myopia among students in underdeveloped regions of China, combined with earlier depletion of the hyperopic reserve, suggests a potential shift toward earlier myopia onset. These findings highlight the need for early interventions in school-aged children, if confirmed through cycloplegic refraction.</p>