Due to an increase in clothes consumption and a decrease in the trend cycle, middle-class consumers generally prefer clothes that are cheap and trendy. The share of global SPA (Specialty store retailer of Private label Apparel) brands in the world fashion market is steadily expanding. However, SPA clothes emphasize trends, resulting in disposal after a season regardless of the quality. Today, the growth of fashion waste raises serious environmental concerns. This study develops sustainable social content for fast fashion. The fast fashion industry has not yet made suitable efforts to achieve sustainability, even though serious environmental pollution is produced by fast fashion. This study investigates the possibility that fashion social content can focus on environmentally friendly products, customer service, fair employment opportunities, fair allocation of profits, diversity of quality and design, and ecosystem promotion. The study provides data to execute a new marketing strategy for sustainable development of global a SPA brand. The qualitative research of the present study was conducted via one-on-one in-depth interviews. The subjects of the qualitative research were individuals who had extensive knowledge about the subject of this research and were employed in the fast fashion and ethical fashion industries. After the qualitative interviews, 200 students were surveyed, and 166 were chosen as research subjects. The results are as follows. First, five sub-themes, including fair-trade storytelling, independent designers, an eco-friendly identity, understanding differences in reform and redesign, and changes in consumer perception, are investigated through in-depth interviews. Second, based on a consumer survey about the intent to participate in different programs, such as 1:1 sponsorship, eco-friendly design sourcing, a reform style contest, redesign consulting, and an eco-fashion gallery, eco-friendly design sourcing programs have the highest intent to participate of all the studied sustainable social programs. The present study is a timely investigation because there has been an increase in the social awareness of benefit sharing. Moreover, consumer perception of the environment is on the rise in South Korea. This study is a pioneer in applying social sustainability, which has until now been used in the fields of environmental, economic, and social research, to fashion social content. The implications for fast fashion firms that care about the ecological consumers are discussed, as are opportunities for further research.