The Pacific oyster, Crassostrea gigas is a globally cultivated species in aquaculture, primarily found in environments with significant temperature fluctuations. However, in recent years, large-scale summer mortality events of oysters have been occurring more frequently worldwide, resulting in significant economic losses to the oyster farming industry and causing severe ecological damage to natural oyster reef habitats. Since 2020, we have initiated an artificial selective backcross breeding program using heat-tolerant wild Crassostrea gigas and Crassostrea angulata as parents, followed by backcrossing with heat-tolerant wild Crassostrea gigas. The aim is to cultivate heat-tolerant backcrossed lines that exhibit phenotypic characteristics similar to wild Crassostrea gigas, in order to improve the summer survival rate of oyster farming.