To examine annual changes in refraction and ocular components around the onset of myopia in Chinese children. Longitudinal study. Twins aged 7 to 15 years in 2006 from the Guangzhou Twin Registry. Participants underwent eye examinations annually from 2006 to 2010. Years were defined (+1 or -1) relative to the examination at which the onset of myopia was first identified. Annual change in spherical equivalent refraction (SER) and ocular biometry. Children who were not myopic at the first examination and myopic in at least 1 subsequent examination from 2006 to 2010 were included in the analysis. Annual change in SER increased slowly from 4 years before the first detection of myopia to 2 years before myopia onset (-0.25 to -0.4 diopter D). The rate of progression was the highest during the year of onset (-0.92 D). After the first detection of myopia, the rate of progression decreased to -0.71 D in the following year and kept decreasing. Annual change in axial length showed a similar, but inverse, shape to that of SER. Annual change in lens power did not change significantly around the onset of myopia. Before the onset of myopia, axial elongation and progression accelerate. After a myopic refraction is established, axial elongation and progression decrease. We suggest that the increases before myopia may be due to increased intensity of study and decreased time outdoors. In contrast, the rapid slowing after the onset of myopia may represent an inhibitory effect of myopic defocus on eye growth.