The original Laminaria polysaccharide (LP0) was sulfated using the sulfur trioxide‐pyridine method, and four sulfated Laminaria polysaccharides (SLPs) were obtained, namely, SLP1, SLP2, SLP3, and SLP4. The sulfated (–OSO3–) contents were 8.58%, 15.1%, 22.8%, and 31.3%, respectively. The structures of the polysaccharides were characterized using a Fourier transform infrared (FT‐IR) spectrometer and nuclear magnetic resonance (NMR) techniques. SLPs showed better antioxidant activity than LP0, increased the concentration of soluble Ca2+ in the solution, reduced the amount of CaOx precipitation and degree of CaOx crystal aggregation, induced COD crystal formation, and protected HK‐2 cells from damage caused by nanometer calcium oxalate crystals. These effects can inhibit the formation of CaOx kidney stones. The biological activity of the polysaccharides increased with the content of –OSO3−, that is, the biological activities of the polysaccharides had the following order: LP0 < SLP1 < SLP2 < SLP3 < SLP4. These results reveal that SLPs with high –OSO3− contents are potential drugs for effectively inhibiting the formation of CaOx stones.