An alkaline phosphatase from green crab (Scylla serrata) was collected from Xiamen seawater in February 1995. It was prepared and purified by means of the following techniques: n-butanol extraction, ammonium sulfate precipitation, ion exchange chromatography on DEAE cellulose column (DEAE-52) and gel filtration chromatography on Sephadex G-200. The preparation was shown to be homogenous on FPLC and polyacrylarnide gel electrophoresis. The characteristic peak of UV-absorption spectrum of the enzyme was found to be at 278 nm, the fluorescence excitation spectrum at 282 nm, and the fluorescence emission spectrum at 343 nm. The molecular weight of the enzyme is 78 kD. At pH = 9.5, the optimum pH is 9.2, at 37°C. At pH = 9.0, 37°C, the Michaelis constant (K_M) is 6.67×10^-4 mol/L; K_cat, 460 min^-1. Magnesium ion activates the enzyme significantly. Copper ion, mercury ion, methanol, ethanol and ethylene glycol inhibit the enzyme activity in varying degrees, which indicates that the conformation of the enzyme caused by the effectors changes at different levels. The inhibition mechanism had been preliminarily studied.