Abstract: The soft soils extensively distributed in the coastal areas of South China are difficult to utilise as resources due to their poor engineering properties. Chemical solidification treatment is an important means for its efficient resource utilization. However, the solidification treatment of coastal soft soil with cement faces the problems of poor solidification effect and high carbon emissions. Therefore, a geopolymer cementitious material was prepared with slag as the precursor and desulfurization gypsum and quicklime as the activators, and the flow solidification of coastal soft soil in South China was carried out. The optimal proportion of the geopolymer cementitious material was determined through unconfined compressive strength tests and microscopic observation tests, and the flow solidification effect of the geopolymer cementitious material on coastal soft soil was evaluated and its microscopic flow solidification mechanism was analyzed. The research results show that the geopolymer of ground granulated blastfurnace slag-desulfurization gypsum - quicklime can effectively flow solidify the coastal soft soil in South China, and its optimal proportion is slag: desulfurization gypsum: quicklime = 80:15:5. Under the optimal proportion, the flowability and 28 d unconfined compressive strength of the geopolymer flow solidified coastal soft soil reach 318 mm and 1.71 MPa, which are 1.1 times and 5.9 times of the cement solidified soil under the same content (14%), respectively. The hydration gel and ettringite of the geopolymer of ground granulated blastfurnace slag - desulfurization gypsum - quicklime can effectively bond the soft soil particles/aggregates, strengthen its microstructure, and thereby improve its flow solidification strength.