Objective To investigate the changes of ocular anterior segment parameters before and after cycloplegia induced by 0.5% tropicamide in children. Methods Three hundred and nine healthy children were selected, cycloplegia was induced by 0.5% tropicamide, and ocular anterior segment parameters were measured with Pentacam system before and after cycloplegia. The changes of ocular anterior segment parameters before and after cycloplegia were investigated, and those stratified by refractive status and genders were analysed. Results The data of 294 children met the inclusion criteria. The central corneal thickness (CCT), corneal volume (CV), anterior chamber depth (ACD) and anterior chamber volume (ACV) after cycloplegia were significantly larger than those before cycloplegia (P<0.05 or P<0.01), and the anterior chamber angle (ACA) after cycloplegia was significantly smaller than that before cycloplegia (P<0.05). The difference between ACD after cycloplegia and that before cycloplegia in children with hyperopia was significantly larger than those in children with emmetropia, low myopia and medium myopia (P<0.05), and the difference between ACV after cycloplegia and that before cycloplegia in children with hyperopia was significantly larger than those in children with low myopia and medium myopia (P<0.001). ACA after cycloplegia was significantly smaller than that before cycloplegia in boys (P<0.05), while ACA after cycloplegia was not different from that before cycloplegia in girls (P>0.05). Conclusion 0.5% tropicamide may cause significant increase in CCT, CV, ACD and ACV, and significant decrease in ACA in children.