Blood urea nitrogen (BUN) is a nitrogenous end product of protein metabolism. This study aims to explore the age- and sex-specific distribution of BUN among healthy Chinese adults. A total of 24,006 BUN values from healthy adults (14,148 males and 9858 females) were included in the cross-sectional study. Males had a higher median BUN value compared to females (4.6 mmol/L vs. 4.1 mmol/L). BUN values showed a positive correlation with body mass index (BMI), cholesterol, and blood sugar (P < 0.0001). However, eGFR showed a negative correlation with the BUN reference value (P < 0.0001) in both sexes. Multiple linear regression analysis confirmed that the positive associations of BUN levels and age were statistically significant after adjusting confounding factors (P < 0.001). Thus, the serum BUN values increased by 0.21 mmol/L for males and 0.282 mmol/L for females per 10 years. The BUN values corresponding to the 1st, 2.5th, 50th, 97.5th, and 99th percentiles for any specific age in both sex were also calculated. These results indicate that the serum BUN reference value is significantly affected by age and gender, and thus, its interpretation is age- and sex-dependent.