•In vitro fermentation behavior of P. haitanensis polysaccharides (PHP) was studied.•PHP could significantly alter the composition and diversity of gut microflora.•PHP could significantly promote the production of short-chain fatty acids.•Molecular weight and intrinsic viscosity of PHP decreased during fermentation.•Degradation products ranged from DP 2 to 9, with the main linkage patterns being 1 → 3 and 1 → 4 linked Galp. This study applied an in vitro fermentation model, whereby the catabolism of Porphyra haitanensis polysaccharides (PHP) was monitored coupled with the variations of microbiota composition and the concentration of short-chain fatty acids (SCFAs), so as to assess the effects of PHP on human intestinal microbiota. After 24 h anaerobic incubation, the level of microflora diversity increased significantly, as the microbiome structure was reshaped through promotion of intestinal probiotics proliferation and inhibition of pathogens growth. Besides, the final concentration of total SCFAs increased to 32.32 ± 1.81 mmol/L, and contained high amounts of acetic, propionic and butyric acid. Furthermore, the molecular weight of PHP decreased from 2.623 × 105 g/mol to 2.308 × 104 g/mol. The degree of polymerization of oligosaccharide products ranged from 2 to 9, with the main linkage patterns being 1 → 3 and 1 → 4 linked Galp. The current study provides new insight on the probiotic activity of PHP within the human gastrointestinal tract.