Abstract: Based on the measured data of COD_Mn in the key monitoring sections of Qiandao Lake, a two-dimensional water quality model for Qiandao Lake based on the Environmental Fluid Dynamics Code (EFDC) Model was established. The influence of inflow COD_Mn from upstream and the inflows around the lake on the COD_Mn in inner Qiandao Lake were analyzed by various scenarios. The response relation between the boundary COD_Mn and the COD_Mn at key monitoring sections in the lake was analyzed, and the influence and contribution of the upstream inflow COD_Mn variation on the COD_Mn of the key monitoring sections in the lake were quantified. Results showed that the comprehensive degradation coefficient (k)of COD_Mn in Qiandao Lake was 0.001 5, according to the parallel simulation of several constant degradation coefficients. There existed an obvious response relation between the upstream inflow COD_Mn and COD_Mn in the key monitoring sections. When the upstream inflow COD_Mn was 6 mg/L, COD_Mn between Weipinglinchang to Santandao sections exceeded Grade Ⅱ water quality standard of Surface Water Environmental Quality Standards (GB 3838-2002). Meanwhile, simulated results showed that the response coefficients of upstream inflows were obviously larger than that of inflows around the lake. The contribution rate of upstream inflows to COD_Mn in key monitoring sections of Weipinglinchang, Xiaojinshan, Santandao and Dabaqian was 82.0%-92.3%, 73.8%-88.2%, 53.8%-62.8% and 57.7%-76.4% , respectively. The impact of COD_Mn on key monitoring sections from upstream inflows was greater than the inflows around the lake. The results indicated that upstream inflows were the main factors affecting the COD_Mn of the key monitoring sections in Qiandao Lake. Therefore, the water quality monitoring and pollutant control should be strengthened for upstream inflows.