In this paper， fucoidan SFP was obtained from Sargassum fusiforme by CaCl2 solution extraction. High performance liquid chromatography （HPLC）， high performance gel permeation chromatography （HPGPC） and chemical methods were used to determine the monosaccharide composition， molecular weight and physicochemical characteristics of fucoidan. Chitosan-fucoidan nanoparticles were prepared using a simple polyelectrolyte self-assembly method （the mass concentrations of fucoidan and chitosan were both 1 mg/mL） and the physicochemical characteristics as well as gastrointestinal stability of the nanoparticles were measured by in vitro simulated digestion. The results showed that the fucoidan from Sargassum fusiforme was mainly composed of mannose， glucosamine， glucuronic acid， galactose and fucose with the molar ratio of 3.96%， 0.37%， 0.42%， 27.20% and 68.05%. The average molecular weight of the fucoidan was about 101.5-701.0 ku. Besides， total sugar， protein and sulfate ester contents were detected and the results were 56.42%， 1.56% and 21.07%， respectively. Uronic acid was not detected using chemical method. The particle sizes of nanoparticles with different ratios of fucoidan/chitosan （Fuc/Cs=0.6 ∶ 1-1.4 ∶ 1） ranged from 330-450 nm and smallest at the ratio of 1.4 ∶ 1 while largest at the ratio of 0.6 ∶ 1. The Zeta potential ranged from 27 to 31 mV， while the PDI from 0.2 to 0.29 indicating that the nanoparticles were stable. The range of fucoidan loading rate was 85%-93%， which decreased with the increasing of Fuc/Cs ratio. Different ratios of nanoparticles showed good stability in the artificial gastric juice with little change in particle size， Zeta potential， PDI and fucoidan loading rate with the extension of time. In the artificial intestinal fluid， the particle sizes of nanoparticles significantly increased. The Zeta potential was all negative and the absolute value decreased while the PDI increased indicating that the nanoparticles swelled and depolymerized. When the ratios of Fuc/Cs were 0.6：1， 0.8 ∶ 1 and 1.4 ∶ 1， the fucoidan loading rate decreased significantly. At the ratio of 1.4 ∶ 1， the fucoidan loading rate decreased to 36%. In summary， Sargassum fusiforme fucoidan /chitosan nanoparticles showed good stability in artificial gastric juice and good depolymerization in artificial intestinal juice at the ratios of 0.6 ∶ 1， 0.8 ∶ 1 and 1.4 ∶ 1， which could be used as a potential delivery system for oral drugs.