Background: Myopia, especially high myopia, would cause damage in the choroid, retina and sclera, thereby leading to vision loss. Although refractive error correction can help improve visual acuity, the pathology of myopia, a global issue, remains unclear and myopia progression, as well as concomitant fundus progression, remains uncontrolled. Under such circumstances, prevention of myopia is of great significance and thus should be prioritized. Objective: To explore whether outdoor time has positive significance for myopia prevention. Search Methods: Databases of Pubmed, Science Direct, the Cochrane Library, the Chinese National Knowledge Infrastructure and the Wanfang Database were searched. The following terms or their combinations were used: myopia, prevention, control, random, randomized, randomization, intervention, outdoor. The full search strategy was shown in the Appendix below. The databases were last searched on October 24, 2018. Selection Criteria: Randomized controlled trials (RCTs) that participants accepted outdoor activity as an intervention measure for myopia prevention were included. Data Collection and Analysis: Two review authors independently extracted data and assessed the risk of bias for included studies. A fixed-effects model was applied, given that the heterogeneity among included studies was small. Main Results: Five RCTs with 3,014 subjects were included. Subjects' age ranged from 6 to 12 years, and the follow-up duration ranged from 9 to 36 months. Spherical equivalent error (SER) of the outdoor group was larger than that of the control group, and the pooled mean difference (MD) was 0.15 (95% CI 0.06-0.23) diopter (D). The change in SER of the outdoor group was smaller than that of the control group, with a pooled MD of 0.17 (95% CI 0.16-0.18) D. New myopia cases in the outdoor group were fewer than that of the control group, and the pooled risk ratio was 0.76 (95% CI 0.67-0.87). The change in axial length of the outdoor group was smaller than that of the control group, and the pooled MD was -0.03 (95% CI -0.03 to -0.03) mm. For all analyzed outcomes, there was no heterogeneity across included studies (I-2 = 0%) and there was no publication bias either. Conclusion: Outdoor time helps slow down the change of axial length and reduce the risk of myopia.