Metsulfuron-methyl is a sulfonylurea herbicide widely used for broad-leaved weed control in cereals. The binding interaction between metsulfuron-methyl and human serum albumin was elucidated by fluorescence, circular dichroism and molecular modeling. The results showed that the alterations of albumin secondary structure in the presence of herbicide induced the slight unfolding of the polypeptide chain of albumin. Fluorescence data revealed that the fluorescence quenching of albumin by herbicide was the result of the formation of the albumin-herbicide complex and hydrophobic and hydrogen bonds interactions were the dominant intermolecular force in stabilizing the complex. Fluorescence probes studies implied that the binding of herbicide to albumin primarily took place in subdomain IIA (Sudlow's site I), and this also corroborates with molecular modeling simulations. This study highlights for the first time the binding mechanism, specific binding site and binding region of herbicide on albumin at the first time. Therefore, this investigation enriches our information of the interaction of sulfonylurea herbicide to the physiologically protein albumin.