Gestational diabetes mellitus (GDM) is a common complication of pregnancy with a complex etiology, and environmental exposure to metals is considered a potential and important risk factor. However, there is still insufficient research on the comprehensive effects of long-term mixed exposure to multiple metals during early pregnancy on GDM and the potential mediating factors. This study aimed to systematically explore the independent and combined effects of exposure to 15 metals during early pregnancy on GDM risk and to clarify the potential mediating role of saturated fatty acids (SFAs) through a prospective cohort of 665 pregnant women recruited between September 2015 and June 2017. Hair samples were collected during early pregnancy, and metal concentrations were measured using inductively coupled plasma mass spectrometry (ICP-MS). SFAs levels in mid-pregnancy were measured by gas chromatography-mass spectrometry (GC-MS). Statistically, logistic regression and restricted cubic splines (RCS) were used to assess the effects of individual metals, and Weighted Quantile Sum (WQS) and Bayesian kernel machine regression(BKMR) model were applied to analyze the combined effects of metal mixtures. Additionally, mediation analysis was conducted to investigate the mediating roles of three SFAs. The results showed that potassium (K), rubidium (Rb), niobium (Nb), silver (Ag), and thallium (Tl) were positively associated with GDM risk, while Nb, Ag, and mercury (Hg) exhibited non-linear exposure-response relationships. Metals mixture had an overall possitive effect on GDM risk. Mediation analysis revealed that myristic acid (C14:0) mediated the associations of K, Rb, Nb, and Ag with GDM risk; pentadecanoic acid (C15:0) mediated the associations of K, Rb, and Nb with GDM risk; and heptadecanoic acid (C17:0) mediated the associations of Rb and Ag with GDM risk.