Black phosphorus (BP) has drawn growing attention as the anode material for lithium-ion batteries (LIBs) because of its high theoretical lithium storage capacity. However, its electrochemical processes and fundamental failure mechanisms have not been completely understood due to the lack of direct evidence. Here, we report the direct visualization of the electrochemical lithiation/delithiation behavior of the BP anode in nano-LIBs using the in situ transmission electron microscopy technique. Upon lithiation, the BP anode is found to undergo obvious anisotropic size expansion and phase change from orthorhombic BP to amorphous Li_xP_y compounds. Unexpectedly, the BP anode pulverizes suddenly during discharging, resulting in irreversibility of the lithiated product and thus poor electrochemical cycling performance. This finding discloses that the failure mechanism of the BP anode is mainly correlated with the delithiation process rather than the lithiation one, which subverts the commonly accepted understanding. The new mechanism insights would serve to provide viable solutions for eliminating rapid capacity fading that plagues the bulk BP LIBs.