Background Heavy metals are widely present in the environment, difficult to degrade, and bioaccumulative. Children's physical development are not mature, and exposure to heavy metals which may cause irreversible harm to them. However, the current research conclusions are still inconsistent. Therefore, it is very important to pay attention to the impact of heavy metal pollutants on physical development. Objective To detect the level of heavy metal exposure of school-age children in Laizhou Wan, Shandong Province, and to explore the relationship between heavy metal exposure level and physical development indexes of school-age children. Methods The subjects of this study were 318 children born to pregnant women recruited from Laizhou Wan Birth Cohort in Shandong Province from September 2010 to December 2013. The school-age children at age 7 were followed up from July to September 2019. Their urine samples were collected and the levels of arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg) in urine were detected by inductively coupled plasma mass spectrometry (ICP-MS). At the same time, the height, weight, body mass index (BMI), waist circumference, and body fat percentage of the children were evaluate to analyze the relationship between the four heavy metals in children's urine and the indicators related to children's physical development. Results Among the 318 school-age children with an age (\begin{document}$\bar x \pm s $\end{document}) of (7.77±0.67) years, the median (P₂₅, P₇₅) levels of As, Cd, Pb, and Hg in urine were 73.98 (44.81, 124.61), 0.40 (0.27, 0.58), 1.20 (0.71, 1.72), and 2.38 (1.32, 4.80) µg·g⁻¹ (of creatinine), respectively. The children’s urinary Cd level was positively correlated with their weight, BMI, and waist circumference (P<0.05), and corresponding regression coefficients (b) and its 95% confidence intervals (CI) were 0.99 (0.14-1.83), 0.43 (0.06-0.81), and 1.35 (0.27-2.42), respectively; while the other heavy metals were not related to the above physical development indicators (P>0.05). After sex stratification, it was found that boys’ urinary Cd level was positively correlated with their height, weight, BMI, and waist circumference (P <0.05) with associated b (95%CI) of 1.73 (0.53-2.93), 2.03 (0.60-3.47), 0.75 (0.11-1.38), and 2.66 (0.85-4.47), respectively; such associations were not found in girls. After further stratification of boys’ BMI according to normal, overweight, and obesity, it was found that a higher urinary Cd level was correlated with an increased risk of obesity in boys (P<0.05), and the associated b (95%CI) was 2.34 (1.02-5.36). Conclusion The level of urinary Cd exposure of boys in Laizhou Wan, Shandong Province is positively correlated with their height, weight, BMI, and waist circumference, and may be related to obesity in boys.