Pseudidiomarina ZJCN121 was cultured by liquid state fermentation. Crude enzyme liquid was separated and purified through acetone precipitation and Sephadex G-100 chromatography with a final fucoidanase purification of 21.8-fold and a specific activity of 20.59 IU/mg and an overall yield of 13.5%. The purified enzyme was identified with molecular mass of 60.2 kDa and isoelectric point of pH 4.3 by SDS-PAGE and EIF-PAGE, respectively. The optimum temperature was 50℃ and the optimum pH 6.5. K_m and V_max for Fucus vesiculosus fucoidan were 5.87 mg·mL^-1 and 6.11 g·L^-1·min^-1, respectively. The fucoidanase was lightly activated by Ca²⁺ and Ba²⁺, and strongly inhibited by Fe²⁺ and Cu²⁺. Its activity also was inhibited by Mn²⁺, K⁺and Zn²⁺, while uneffected by Mg²⁺.