With the rapid development of low-dimensional materials, the opportunity that promotes the development of micro/nano fluid devices, a new low-dimensional material black phosphorus (BP) has attracted wide attention due to its excellent properties, and has been applied to many areas. In this paper, the influences of driving force, water-BP anisotropy, channels’ width and the number of black phosphorus layers on the flow characteristics of water molecules in the nanochannels are studied by molecular dynamics based on the Poiseuille flow model. The results show that the boundary slip velocity increases with the driving force increasing. The anisotropy will also affect the flow characteristics of water molecules in the nanochannel under the pressure driving the Poiseuille flow. Specifically, the boundary slip velocity decreases with the chirality angle increasing, and the viscosity coefficient of water molecules is still not affected by the anisotropy. The natural rippled structure of the BP surface leads to the coarse potential surface, and further results in the anisotropic boundary slip and interfacial friction between water and BP sheets. With the driving acceleration kept constant, the influences of the width of nanochannels and the number of black phosphorus layers on the boundary slip velocity and viscosity coefficient of water molecules are investigated. The results indicate that the slip velocity of water molecules in the nanochannels decreases with the width of the nanochannels increasing. The velocity profile of water molecules in the bilayer model is slightly different from that in the monolayer model. With the number of BP layers increasing, the energy of BP-water solid-liquid interface increases while the anisotropic interfacial property remains unchanged. The results will provide a theoretical basis for the study of the characteristics of the fluid flowing in the black phosphorus nanochannels and the design of micro/nano fluid devices based on black phosphorus materials.