Obstructive sleep apnea (OSA) is commonly observed in children with Down syndrome (DS) and may affect their physical and psychological development. Currently, adenotonsillectomy is the first line treatment option for paediatric patients with OSA. However, surgical outcomes for such patients are not satisfactory. In this study, we analysed the efficacy and safety of adenotonsillectomy in the treatment of children with obstructive sleep apnea and Down syndrome. We systematically searched the PubMed, Web of Science, EMBASE, and the Cochrane databases and pooled data from nine relevant studies involving 384 participants. Subsequently, we analysed four outcomes in polysomnography, namely: net postoperative changes in the apnea–hypopnea index (AHI), the minimum oxygen saturation, sleep efficiency, and arousal index. Meta-analysis of the AHI showed a decrease of 7.18 events/h [95% CI (−9.69, −4.67) events/h; p < 0.00001] and an increase in the minimum oxygen saturation of 3.14% [95% CI (1.44, 4.84) %; p = 0.0003]. There was no significant increase in sleep efficiency [MD 1.69%, 95% CI (−0.59, 3.98) %; p = 0.15], but the arousal index significantly decreased by −3.21 events/hour [95% CI (−6.04, −0.38) events/h; p < 0.03]. In addition, the overall success rate was 16% (95% CI, 12%–21%) for postoperative AHI < 1 and 57% (95% CI, 51%–63%) for postoperative AHI <5. The postoperative complications recorded included airway obstruction and bleeding. This study demonstrated the efficacy of adenotonsillectomy as a treatment option for OSA. However, it is important to note that residual OSA and potential postoperative complications require further attention in future studies.