Background It is generally believed that a 1‐mm axial length (AL) elongation of the eye corresponds to a −3.00 D spherical equivalent (SE) progression, but this is disputed. Purpose To investigate the association between AL elongation and SE progression among children and adolescents. Methods A prospective cohort study of 710 children and adolescents aged 6–16 years was included. Ophthalmic examinations, including cycloplegic SE, AL and corneal curvature, were performed at baseline and 1‐year follow‐up. The ratio of SE change (ΔSE) to AL change (ΔAL) (ΔSE/ΔAL) was calculated, and its association with age and refractive status was explored using a general linear model. Results Among all participants, 396 (55.77%) were male, with 265 (37.32%) myopes at baseline. The average 1‐year ΔSE and ΔAL were 0.61 ± 0.40 D and 0.33 ± 0.22 mm, respectively. Both ΔSE and ΔAL gradually decreased with age (p < 0.001). In the general linear model analyses, age and refractive status were independently associated with ΔSE/ΔAL after adjustment for covariates (age: trueβ̂$$ \hat{\beta} $$ = 0.04, p < 0.05; myopia vs nonmyopia: trueβ̂$$ \hat{\beta} $$ = 0.28, p < 0.05). Based on the developed formula ΔSE/ΔAL = 1.74 + 0.05*age (for myopes), mean ΔSE/ΔAL in myopes increased from 2.06 D/mm in the 6‐year‐olds to 2.59 D/mm in the 16‐year‐olds. In nonmyopes, ΔSE/ΔAL = 1.33 + 0.05*age, and the ratio increased from 1.65 D/mm in the 6‐year‐olds to 2.18 D/mm in the 16‐year‐olds. Conclusions The ratio of ΔSE/ΔAL varied with age and refractive status in children and adolescents. The age‐specific ΔSE/ΔAL could be used to estimate SE progression through the actual AL change.