The invention relates to a new 
bilobalide B compound. The 
bilobalide B compound is extracted from 
ginkgo leaf; and the 
bilobalide B content in the bilobalide B compound is greater than 99%. The bilobalide B compound contains bilobalide A of which the content is lower than 0.4%. The invention also relates to a preparation method of the bilobalide B compound, which comprises the following steps: under stirring conditions, adding kieselguhr and 
ginkgo leaf extract, heating under 
reflux at 78-85 DEG C for 0.5-1.5 hours, filtering and concentrating; adding 
distilled water, removing impurities, separating out the 
ethyl acetate layer, passing the 
ethyl acetate layer through an peracidic aluminum 
oxide column, eluting with water saturated 
ethyl acetate, and collecting the eluate; concentrating the eluate under reduced pressure until no ethyl acetate smell is emitted, adding a proper amount of 
ethanol to dissolve the concentrated eluate, adding water until the 
ethanol content is 60-70%, standing, precipitating coarse 
crystal, filtering, taking the coarse 
crystal, and 
drying; and recrystallizing the coarse 
crystal sequentially with 
ethanol and ethyl acetate to obtain the finished product bilobalide B compound.