The scheme of a frequency control system in a rotary drill is established by integrating a converter and a programmable logic controller (PLC). The principle of speed regulation, characters of speed-up and speed-down, and the mechanical running performance are also analyzed. The result of application indicates that the frequency control system excellently solves the problems of start, stop, and speed regulation of the drill. The equipment maintenance workload and cost are thus reduced. The scheme of a frequency control system in a rotary drill is established by integrating a converter and a programmable logic controller (PLC). The principle of speed regulation, characters of speed-up and speed-down, and the mechanical running performance are are analyzed . The result of application that that the frequency control system excellently solves the problems of start, stop, and speed regulation of the drill. The equipment maintenance workload and cost are therefore reduced.