Basalt fiber (BF) was blended into phosphogypsum-based composites (PGC), and the effects of BF diameter, length and content on the durability of PGC were investigated. The results show that the BF addition can significantly reduce the corrosion ratio of PGC. With the increase of BF content, the strength of samples in wet-dry cycle and freeze-thaw cycle increases generally, and the change mechanism is basically consistent with that of the absolute dry strength. The compressive strength and flexural strength in wet-dry cycle increase by 22.3% and 100.3% compared with blank group, respectively. The compressive strength and flexural strength in freeze-thaw cycle increase by 46.5% and 124.0%, respectively. At the same time, the dry-wet cycle and freeze-thaw cycle strength coefficients of PGC increase with the increase of BF content. The dry-wet cycle compressive strength and flexural strength coefficients increase to 095 and 092, and the freeze-thaw cycle compressive strength and flexural strength coefficients increase to 071 and 062, respectively, indicating that the durability of PGC has been significantly improved. In addition, the diameter of BF has no significant effect on the durability of PGC. The results provide a reference for the study of durability of fiber modified gypsum-based composites.