Abstract: By electron beam deposition of 1 nm Sn as a buffer layer and deposition of hafnium dioxide(HfO₂)as the gate dielectric of black phosphorus(BP) top-gate devices(TG-FETs), the mobility of the prepared BP TGFETs can reach more than 100 cm²V^-1s^-1. By testing transfer characteristic curves of the BP TG-FETs under different drain bias voltages, it is found that the drain bias voltage causes the gate control effect. Furthermore, the sourcedrain current saturation of BP top-gate TG-FETs with different channel lengths under a small voltage is explored,which can be used to study BP radio frequency devices. Finally, the quantum capacitance of the BP TG-FET was studied. The oxide layer capacitance and BP quantum capacitance were directly calculated from the measured CV curves and the transfer characteristic curves without introducing any other fitting parameters. The measurement of BP quantum capacitance also provides an important basis for exploring the density of states of BP and the compressibility of BP electronic devices.