A controllable oscillating system for generating a differential oscillating signal is disclosed. The controllable oscillating system includes an oscillating circuit and a current adjusting device. The oscillating circuit includes a controllable resonator, a cross-coupling driving device, and a current source. The cross-coupling driving device is coupled to the controllable resonator and utilized for driving the controllable resonator to generate the differential oscillating signal. The current source is coupled to the cross-coupling driving device and utilized for providing a first current. The current adjusting device is coupled to the cross-coupling driving device and utilized for adjusting currents passing through the cross-coupling driving device.