A signal swing trimming apparatus and a method thereof. The signal swing trimming apparatus calibrates a swing level of an output signal generated from a transmitting device to a receiving device including: a comparing device coupled to the output signal for comparing the swing level of the output signal with a target swing level and generating a comparison output signal, and an adjusting device coupled to the comparing device and the transmitting device for controlling the transmitting device to adjust the swing level of the output signal according to the comparison output signal, wherein the signal swing trimming apparatus is configured to calibrate the swing level of the output signal during a hand-shake process between the transmitting device and the receiving device. The invention solves the compatibility problem owing to mismatching of the resistance.