Abstract: Objective: This study endeavors to synthesize selenium nanoparticles (SPS-SeNPs) by low molecular weight Porphyra haitanensis polysaccharide, with a primary focus on assessing their morphological attributes and antioxidant efficacy. Methods: Polysaccharide concentration, reaction time, and reaction temperature were systematically manipulated to optimize the preparation process. The preparation process of SPS-SeNPs was systematically investigated through both single-factor and orthogonal experiments. The determination of SeNPs particle size within SPS-SeNPs was executed via dual-wavelength colorimetry, with the absorbance ratio (A_410/490) of the SPS-SeNPs solution serving as a pivotal evaluation metric. Structural elucidation encompassed scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The investigation extended to the assessment of total antioxidant activity, reducing power, and DPPH free radical scavenging rate. Results: The optimal conditions for SPS-SeNPs preparation were ascertained at a polysaccharide concentration of 2 mg/mL, a reaction temperature of 40 ℃, and a reaction time of 0.5 h, resulting in an absorbance ratio of 2.0487±0.0082. SeNPs within SPS-SeNPs exhibited a well-dispersed spherical morphology. The elemental composition percentages of SPS-SeNPs were found to be as follows: C (42.00%), O (41.35%), and Se (16.65%). The selenium within SPS-SeNPs was validated to be in an amorphous state with a zero valence, maintaining stability over 30 days at 4 ℃ under dark storage conditions. It demonstrated outstanding total antioxidant capacity, reducing power, and DPPH free radical scavenging ability (IC₅₀=31.45±1.16 µg/mL), surpassing the performance of sodium selenite (Na₂SeO₃) and low molecular weight P. haitanensis polysaccharides (SPS). Conclusions: SPS-SeNPs synergistically harness the antioxidative properties of polysaccharides and the heighten biological activity of nano-selenium, thereby furnishing substantial theoretical underpinning for the progressive development and application of crude P. haitanensis polysaccharides.