Abstract: We investigated the seawater and sediments in a deep-water net cage area in Dadui, southwest Daya Bay, to assess the impact of farming activities on the ambient environment. The water and sediment samples were collected from cage culture area, surrounding area (located ca. 0.1 km from the nearest farming cage) and non-culture area (where no culture activities were practiced, located ca. 10~15 km from the nearest farming cage) in May (before the farming) and August 2016 (the most feeding activities and metabolism of cultured fish), respectively. The water environment was assessed using organic pollution index (A) and nutrient quality index (NQI), while the heavy metals potential ecological risk in surface sediment was assessed using potential ecological risk index (RI). The results show no significant difference of COD_Mn concentration among the three areas of deep-water net cage farming, while COD of the traditional cage farming showed a decreasing trend from the center to the surrounding of the cage area. The A value ( < 1) of cage culture area indicates that the water quality was in a general condition, and the relatively low NQI value ( < 2) indicates that the water quality was in a relatively poor nutrient condition. The contents of heavy metals in surface sediments of cage area were lower than the marine sediment quality standards ClassⅠexcept for those of Cu and Cd. The E_rⁱ value ( < 30) and RI value ( < 100) were relatively low, compared with many other culture areas in China, which indicates that the surface sedimentary environment of deep-water net cage area in Daya Bay is in slight ecological harm. It is suggested that the influence of deep-water net cage fish farming on the ambient water and sedimentary environment is relatively low or acceptable.