The molecular weight is a fundamental property of dissolved organic matter (DOM) that affects the fate of arsenic (As) in groundwater. However, there is limited knowledge regarding the various molecular weights on the geochemical transformation mechanisms of DOM with respect to antimony (Sb) migration in groundwater. A total of 20 samples were collected from high- and low-Sb D3x4 waters in the world’s largest antimony mine to evaluate the effects of different molecular weights of DOM on Sb mobilization using a sequential ultrafiltration technique. Dissolved Sb occurred mainly in the <1kDa fraction, while total Fe (TFe) colloids and DOM mostly existed in <0.45-μm and <100-kDa fractions, respectively. A protein-like component with a higher biological index (BIX), lower humification index (HIX), and specific ultraviolet absorbance (SUVA254) demonstrated a higher binding potential to Sb. Owing to the lower values of δ13CDIC a nd t he d ifference b etween δ13CDIC a nd δ13CDOC, the microbial degradation of DOM had a substantial contribution to Sb mobility in the D3x4 water. The results obtained from this research contribute to our comprehension of the biogeochemical behavior of antimony in shallow groundwater.