In order to explore the key marine environmental factors affecting the growth and fattening of triploid Crassostrea angulata and promote the development of triploid oyster culture to the deep waters in the outside bay, a 10-month survey was conducted on the triploid C. angulata culture area in Huangqi Bay from December 2021 to September 2022. The growth indexes of oysters and marine environmental factors of the culture areas both outside and inside the bay were obtained, and the data were analyzed by Redundancy analysis （RDA） and other methods. The results showed that there were significant differences in growth indexes of oysters such as shell length, shell width, total mass, soft tissue mass, and fillet yields between the inner and outside bay groups, with all those indexes being significantly higher in the outside bay than those in the inner bay, while the difference of shell heights inside and outside bay was not significant. The contents of nitrate nitrogen, chlorophyll-a and phytoplankton cell density in the seawater in the outside bay were significantly higher than those in the inner bay, and there was no significant difference between other indexes inside and outside the bay. The results of RDA analysis showed that the main factors affecting the growth indexes of oysters were water temperature, nitrate nitrogen, phytoplankton cell density and chemical oxygen demand. What's more, phytoplankton cell density and chlorophyll-a content had the greatest effect on oyster fillet yield. Comprehensive analysis showed that water temperature, phytoplankton biomass and nutrients were the key marine environmental factors affecting the growth and fattening of triploid oysters. The average chlorophyll-a content in the sea area was above 6 μg/L for at least one month from spring to early summer, which was beneficial to the fattening of oysters. The selection of non-pollution source, good water exchange conditions, abundant food, rich nutrient supply, stable salinity and low wind waves in the outside bay to cultivate triploid oysters may be helpful to improve the yield and quality of oysters.