Abstract: The fiber reinforced cement composite blended with phosphogypsum solid waste was successfully prepared using the flow slurry method. Firstly, the solid waste phosphogypsum was neutralized with quicklime, decontaminated and baked to stimulate its cementitious properties, and then mixed with mineral powder and steel sand in the ratio of 5∶4∶1 to form modified solid waste phosphogypsum mix (MWPM) to replace part of the cement as an auxiliary cementitious material. At the same time, solid waste phosphogypsum aggregate was used to replace part of the natural fine aggregate, while a small amount of polyoxymethylene (POM) fibers and wood pulp fibers were considered to be incorporated to carry out a baseline proportion design study of fiber-reinforced cementitious composites and to analyses their strength and micro-mechanisms in combination with XRD and SEM. The results show that the dosage of MWPM with phosphogypsum aggregate has almost no effect on the density, water absorption, thermal conductivity, etc. of solid waste fiber-reinforced cement boards; Although the strength in the early stage decreases slightly with the increase in the dosage of MWPM, it can still be maintained at about 4 MPa when the dosage reaches about 18%, and the strength in the later stage is partially enhanced compared to that of MWPM, while the ratio of flexural strength is not less than 70% after freeze-thaw test, which meets the specification requirements of non-load-bearing fiber reinforced cement board. Phosphogypsum aggregates have a minor reinforcing effect on the composites, which is relatively optimal at 20%; Based on the above research, a benchmark mix ratio for fiber-reinforced cement-based composite materials incorporating phosphogypsum waste has been proposed. This mix ratio effectively utilizes phosphogypsum waste at 155.39 kg/m³, reduces cement consumption by 78.58 kg/m³, and lowers carbon dioxide emissions by 27.60 kg/m³. Microscopic analysis reveals that dihydrate phosphogypsum, ettringite, and C-S-H gel interpenetrate to form a three-dimensional spatial structure. Additionally, the incorporation of POM fibers and wood pulp fibers enhances the integration of the matrix, aggregates, and the two types of fibers, creating a denser overall structure. This structure further improves the mechanical performance of the specimens. The fiber-reinforced cement-based composite material with mixed phosphogypsum waste demonstrates excellent mechanical properties and waste utilization efficiency, providing a reference for the development of non-load-bearing green insulation materials.