Charybdis japonica is an important marine crab widely distributed in China sea area.The main purpose of the study was to provide a theory basic for the research of reproduction biology and promote the progress of artificial breeding technolagy of C.japonica.The morphological changes of sperm during the acrosome reaction in C.japonica were studied in detail with light and transmission electron microscope, and the optimal induced conditions of acrosome reaction in vitro was obtained by orthogonal experiments.Then the induced effect of ionophore A23187 and egg-water was investigated by contrast experiment,and the acrosome reaction rate of the sperm collected from male seminal receptacle has been analysed induced by treatment with ionophore A23187.The process of acrosome reaction consists of four consecutive stages:(1) swelling of the apical cap,(2) the apical cap ruptures to form an aperture which acrosomal vesical can evert through it and radial arms contract,(3) elongation forward of perforator and aggregation of acrosomal granule in the posterior segment of acrosomal tubule,(4) rupture of perforator and shedding of the acrosomal vesicle outer layer result in formation of the acrosomal filament,then acrosomal granules release out and nuclear size reduce.Most of the acrosome reaction of C.japonica sperm were complete in 50 minutes.Three factors with five levels including a series of pH(6.0,7.0,8.0,9.0 and 10.0),a series of different concentrations in Ca² (0.10%,0.15%,0.20%,0.25% and 0.30%)and A23187(30,40,50,60 and 70 μg/mL)were chosen to assay the optimal induced conditions for the acrosome reaction in sperm of with an orthogonal experiment.The highest rate(80.41%)of the acrosome reaction in sperm collected from female spermatheca can be achieved when sperm exposed to 60 μg/mL A23187,0.30% CaCl₂ at pH=8.0 in Ca² free artificial seawater for 50 minutes,but the acrosome reaction rate of the sperm is 73.87% induced by egg-water.64.37% of acrosome reaction rate can be achieved by treating the sperm taken from male seminal receptacle with the optimal induced conditions.