There is growing evidence that vitamin D is related to the development of a variety of diseases. The current study was performed to investigate the status of serum vitamin D distribution among adult Chinese people and reveal the influence of gender, age, seasonality and residential regions on serum vitamin D levels. This cross-sectional study included 14,302 participants aged from 18 years old to 65 years old from six major cities in China. The basic demographic information and the levels of serum vitamin D (25(OH)D) and vitamin D3 (25(OH)D3) were collected from Jan 2, 2014 to Dec 25, 2017. The prevalence of 25(OH)D3 concentration <30ng/mL reached up to 83%, in which the rate of vitamin D insufficiency (20-29ng/mL) was 32.7%, and vitamin D deficiency (10-19ng/mL) accounted for 41.9%, and vitamin D severe shortage (<10ng/mL) accounted for 8.4%. Women were more likely to have vitamin D3 deficiency and lower serum vitamin D3 concentration than men (both p<0.001). The mean concentration of serum 25(OH)D and 25(OH)D3 in summer and autumn were higher than that in spring and winter (p<0.001), and the mean concentration of serum 25(OH)D in people from Southern China was higher than that in people from other regions (p<0.001). Although the mean concentrations of serum 25(OH)D and 25(OH)D3 were both increased by age, the percentage of patients with serum 25(OH)D3 insufficiency was also increased. Serum vitamin D deficiency is very common in adults in China. The level of serum vitamin D may be associated with age, sex, seasonality and residential regions. Copyright © 2019 Elsevier España, S.L.U. All rights reserved.