The composition, structure and bioavailability characteristics of dissolved organic nitrogen (DON) in the water-land interface of Erhai Lake were studied by using fluorescence and UV-vis absorbance. Moreover, the effect on water quality was also discussed. Results showed that: (1) The DON concentration of the water-land interface from Erhai Lake via Yongan River was between 0.25 and 1.39 mg·L-1, and the DON concentration were relatively high by summer and autumn from the inflow river input into the Erhai Lake. The loading of DON into the Erhai Lake by Yongan River between 0.48 and 2.34 t, and it was relatively high in summer, suggesting that the potential risk for impact on the water quality of Erhai Lake was relatively large by the inflow river into the Erhai Lake, especially in the summer. (2) The DON mainly comprised humic-like substances in the water-land interface from the Yongan river flow into the Erhai Lake, showing the decrease by the microbial degradation with 15.27%, however, aromatic proteins-like and dissolved metabolites of the microorganisms were accumulated by 42.00% and 20.68%, respectively to some extent. In addition, the humification degree of DON was gradually decreased by 14.97% via Yongan River into Erhai Lake. Moreover, the substitution groups in the aromatic rings of DON showed that the transformation of mainly contained aliphatic chains to carbonyl, carboxyl, hydroxyl and esters from inflow river into Erhai Lake, particularly in the summer; (3) The composition characteristic parameters of DON (P(Ⅲ+Ⅴ, n)/P(Ⅰ+Ⅱ, n), A253/A203) was positively correlated with the contents of the different nitrogen (N) forms (R2=0.64～0.74, p<0.1, p<0.05) in the water-land interface from Erhai Lake via Yongan river, that was, the structure affect their bioavailability; the different structural characteristic parameters of DON in the water-land interface from Erhai Lake had a significant positive correlation with water quality(R=0.82～0.96, p<0.05, p<0.01), indicating that the composition, structure and bioavailability of DON had contributed to the water pollution in the water-land interface from the Yongan river input into the Erhai lake, especially being greatly affected by the terrestrial pollution such as human activity in summer. Therefore, the change of composition and structure of DON (such as P(Ⅲ+Ⅴ, n)/P(Ⅰ+Ⅱ, n), A253/A203) in the water-land interface from the Yongan river into lake could reflect the condition of the water quality of Erhai Lake to some extent.