Objective: To investigate the mediation effect of glucose level on the linking between maternal age and offspring birthweight. Design: Cohort study Setting: Single tertiary center Population: Women who participated in the National Free Preconception Health Examination Project from January 2015 to September 2017 in Dongguan city, China. Methods: A causal mediation analysis was applied to test the potential mediation effect of the glucose level on the association between maternal age and offspring birthweight. Main Outcome Measure: Offspring weight, macorosmia Results: Of 12 044 women with singleton birth were included. The mean birthweight was 3163.12 ± 417.20 g, the frequency of macrosomia was 1.9%, and 1.8% women were in a hyperglycemic state. Mothers aged 30 years and over were significantly more likely to have preconception hyperglycemia [OR (95% CI): 1.82 (1.31, 2.52)]. Both maternal age and preconception glucose level had a significant positive linear association with macrosomia after adjusted potential confounding factors [OR (95% CI): 1.66 (1.22, 2.26), 1.30 (1.14, 1.47), respectively]. The mediation analysis showed that the presence of preconception glucose level mediated significantly 8% [95% CI: 4% to 16%] of the total influence of maternal age on offspring birthweight. Additionally, the mediated effect was increased among women with a history of adverse pregnancy by mediating 18%. Conclusion: The preconception glucose level mediated the association of advanced maternal age with offspring birthweight. It suggested that the importance of preconception glycemic monitoring and control among older mothers to reduce the risk of adverse pregnancy outcome.