Alginate fiber, a natural biomass fiber, has garnered significant attention due to its exceptional biocompatibility, degradability, and mechanism for metal ion flame retardation. However, the smoldering phenomenon leads to inadequate flame retardant effects of alginate fiber in applications. Additionally, the current preparation process of alginate fiber solely relies on alginate, leading to the insufficient utilization of other valuable resources in seaweed, such as cellulose and polypeptides. This inefficiency also results in resource wastage and environmental pollution, primarily due to alginate production's high energy and water consumption. In this study, we employed a straightforward co-hybrid wet spinning technique to fabricate all-seaweed biomass flame retardant composite fiber, incorporating seaweed polypeptides and calcium alginate. This approach significantly mitigated smoldering. Analysis of the thermal decomposition process showed that the composite fiber displayed a metal-ion/nitrogen synergistic flame retardant effect in the condensed phases, contributing to the safety of composite fiber. This research provides a more environmentally friendly solution for improving the fire safety of metal-ion flame retardant systems in applications while increasing the fficiency of seaweed resource utilization and reducing the environmental impact of the seaweed extraction process.