Abstract: The runoffs from irrigation and drainage ditches are the major sources of nitrogen and phosphorus of Erhai Lake. In the Autumn of 2020, two catchments in the western Erhai Lake Basin were selected to analyze the change characteristics of ditch water quality in catchment scale and sub catchment scale under the rain condition, as well as the impact of land use on ditch water quality. The results indicated that the water quality of the ditches presented obvious spatial variation. In Shuangying catchment, the chemical oxygen demand (COD) of the ditch runoff was relatively high in the southeast, which ranged from 5.68-20.49 mg/L, and that of total nitrogen (TN) of the ditches in the north was relatively high, which ranged from 1.04-1.44 mg/L. The total phosphorus (TP) concentration decreased from east to west, and ammonia nitrogen (NH₃-N) concentration was generally low in the catchment. In Wanyang catchment, the COD concentration of the ditch runoff was relatively high in the south, which ranged from 5.02-14.22 mg/L, and the TN concentration was generally low, and the TP and NH₃-N concentrations were generally high in the ditches around Erhai Lake and north of the central part. The redundancy analysis indicated that the farmland and village were the major source of TP and NH₃-N in the ditch runoff. The unutilized water storage in ponds and wetlands may cause the ditch water quality pollution in the Autumn. The grass and forest land area presented negative relationship with the ditch water quality, but due to the low proportion and spatial location, it had little effect on reducing the pollution load into the sea. To reduce the impact of irrigation and drainage ditch backwater and rural domestic sewage on Erhai Lake, measures should be taken to strengthen the recycling of irrigation water, and improve the sewage interception project around the lake.