To explore the relationship between the level of serum 25-hydroxyvitamin D (25-(OH) D) of pregnant women with gestational diabetes mellitus (GDM) during the second trimester of pregnancy and the levels of their insulin like growth factor-1 (IGF-1) and inflammatory factors. Methods: The pregnant women with GDM were selected and divided into group A (women with serum 25-(OH) D <50nmol/L), group B (women with serum 50≤25-(OH) D <75nmol/L), and group C (women with serum  25-(OH) D ≥75nmol/L) from June 2020 to April 2021. The fasting venous blood of the women in the three groups during 24-28 gestational weeks was collected for detecting the levels of their IGF-1, interlleukin-6 (IL-6), and tumor necrosis factor-A (TNF-A). Results: The levels of IL-6 and TNF-A of the women in group A and group B were significantly higher than those of the women in group C, and which of the women in group A were significantly higher than those of the women in group B (P<0.05). The expression of IGF-1 level of the women in group A was significantly lower than that of the women in group B and group C (P<0.05), but which of the women had no significant different between group B and group C (P>0.05). The levels of IL-6 and TNF-A of the women in group A and group B were negatively correlated with their 25- (OH) D level (r =-0.71-0.58, all P<0.05), and the IGF-1 level of the women in group A was positively correlated with their 25- (OH) D level (r=0.51, P<0.05). Conclusion: The deficiency and deficiency of Vitamin D of the pregnant women during the second trimester of pregnancy are related to their GDM occurrence, and its pathogenesis may be related to the regulation of the IGF-1 level and the secretion disorder of inflammatory factors of IL-6 and TNF-A.