A system for controlling volume with a single knob includes: a knob (10) for generating rotation signals; a controlling apparatus (11) for determining a rotational direction and speed of each rotation signal, and generating corresponding volume controlling signals for adjusting volume according to the rotation signals; and an audio amplifier (12) for adjusting volume according to the volume controlling signals. The knob includes a rotational operation part (100) and a rotation signal generating circuit (101). The controlling apparatus includes a number of input ports (110), a timer (111), a counter (112), a ROM (Read Only Memory) (113), a RAM (Random Access Memory) (114), a CPU (Central Processing Unit) (115), a PWM (Pulse Width Modulator) (116), and a number of output ports (117). A related method for controlling volume with a single knob is also disclosed.