Black phosphorus (BP) is a kind of direct bandgap 2D semiconductor with natural fold structure. It has attracted lots of attentions since 2014 for its outstanding physical properties. In this article, different-sized 2D BP nanosheets were successfully prepared by liquid exfoliation. The sizes, morphologies and thicknesses were characterized by scanning electron microscope, transmission electron microscope and atomic force microscope. Then the size-dependent Raman scattering spectra were collected. The biodegradability provides a possibility of BP for the biomedical application. Thus, degradation characteristics of BP nanosheets in aqueous solution were investigated. The time-dependent absorption spectra and pH revolution both suggest that the smaller BP sheets degraded faster, the predominant degradation occurred in the incipient 3 days. The underlying mechanism is attributed to the generation of orthophosphoric acid or other phosphorous acid.