Plant polysaccharides’ chemical structures are similar to that of glucosaminoglycan, a stone inhibitor in urine; thus plant polysaccharides may be used to prevent and treat kidney stones. However, the applications of natural polysaccharides are limited due to their large molecular weights and volume. In this study, the effects of four degraded Gracilaria lemaneiformis polysaccharides (GLP1, GLP2, GLP3 and GLP4) with molecular weights of 49.6, 16.2, 8.2 and 3.8 kDa, respectively, on crystal growth of calcium oxalate (CaOx) were studied. 1H NMR, 13C NMR and gas chromatography-mass spectrometry (GC-MS) spectra showed that the four GLPs were composed of β-D-galactose and 6-O-sulfate-3,6-α-L-galactopyranose. X-ray diffraction (XRD) patterns showed that GLPs induced the formation of calcium oxalate dihydrate (COD) crystals, and the diffraction peaks of COD appeared at crystal plane spacing d=0.617, 0.441, 0.277 and 0.224 nm. However, only calcium oxalate monohydrate (COM) crystals were formed in the absence of polysaccharide, and the diffraction peaks of COM appeared at d=0.593, 0.364, 0.296 and 0.235 nm. Because COD is more easily excreted out from the body than COM, COD formation is beneficial to reduce the risk of stone formation. Fourier-transform infrared spectroscopy (FTIR) showed that the asymmetric stretching vibration νas(COO-) and symmetric stretching vibration νs(COO-) of carboxyl groups from oxalate had different degrees of blue shift as the molecular weight of GLP decreased or GLP concentration increased, in which νas(COO-) increased from 1 618 to 1 642 cm-1 and νs(COO-) increased from 1 318 to 1 328 cm-1. That is, GLP4 induced full COD crystals. Scanning electron microscope (SEM) examination showed that as the molecular weight of GLP decreases, the proportion of COD increased, and the dispersion degree of the crystal increased, making the crystal much blunter. As GLP molecular weight decreased or GLP concentration increased, the surface charges of CaOx crystals induced by GLPs changed to be more negative. The greater absolute value of Zeta potential is conducive to inhibiting crystal aggregation. The inductively coupled plasma emission spectra (ICP) results revealed that the four GLPs increased the concentration of soluble Ca2+ ions (c(Ca2+)) in the solution and decreased the amount of CaOx precipitation. At a concentration of 1.0 g·L-1, the c(Ca2+) in the supernatant were GLP4 (37.88 μmol·L-1)>GLP3 (19.70 μmol·L-1)>GLP2 (16.05 μmol·L-1)>GLP1 (10.55 μmol·L-1), respectively. The four GLPs can inhibit COM growth, induce COD formation, reduce the aggregation degree of crystals, increase the absolute value of Zeta potential on crystal surface and the concentration of soluble Ca2+ ions in solution, and reduce the amount of CaOx crystal formation. The regulatory ability of GLP was negatively correlated with its molecular weight. All these results showed that GLPs, especially GLP4 with the smallest molecular weight, might be potential drugs for the preventing and treating CaOx stones.