A new type of green building materials named alkali-activated slag cementitious materials with seawater and sea sand (AASCM-SS) was proposed. AASCM-SS can effectively utilize waste slag, especially suitable for emergency repair projects in coastal areas. To promote its engineering application, the mechanical properties of AASCM-SS at room temperature and high temperature were studied. The influence of polypropylene (PPFs) and polyvinyl alcohol fiber (PVA) on mass loss rate, deformation, compressive strength, flexural strength, and micromorphology of AASCM-SS after high temperature were studied. The research results show that PPFs and PVA can hardly improve the compressive but can slightly improve the flexural strength of AASCM-SS at room temperature. Besides, PPFs and PVA can significantly improve the high temperature resistance of AASCM-SS, with the best effect at temperatures ranging from 200 ~ 400 °C. Moreover, adding PPFs and PVA reduced the deformation of AASCM-SS after high temperature. At last, the channels formed after the melting of PPFs and PVA reduced the internal vapor pressure of AASCM-SS, thereby improving its high temperature resistance.