As an important glycoprotein of the lectin family, soybean agglutinin (SBA) is an anti-nutritional factor with considerable toxic and side effects and plays a significant role in tumor analysis. In order to achieve the sensitive detection of SBA, a sandwich-structured electrochemiluminescence (ECL) biosensor was constructed using carboxylated carbon nitride (C-g-C 3 N 4 ) as luminophore and D-galactosamine (galM) as a recognition element. A glassy carbon electrode (GCE) was modified with Au nanoparticles (Au NPs) for capturing the galM via Au-N bond, and further capturing the target SBA by specific recognition between galM and SBA. In the presence of SBA, the composite C-g-C 3 N 4 -galM was immobilized onto the electrode. With the increase in the concentration of SBA, the ECL signal from C-g-C 3 N 4 increased, thus achieving a signal-on detection of SBA. The linear range of the biosensor was 1.0 ng/mL~10 μg/mL and detection limit for SBA was as low as 0.33 ng/mL. In this construction strategy, C-g-C 3 N 4 not only acted as an excellent signal probe, but also as an immobilization matrix to easily achieve a high loading of the small molecule recognition element galM. This strategy provides a simple alternative SBA detection platform. Graphical abstract