The invention discloses a stylus pen, a touch sensing system and a driving method thereof. An active stylus pen generates a pen driving signal synchronized with a touchscreen driving signal received from a touchscreen and outputs the pen driving signal to the touchscreen. The active stylus pen includes a pressure sensing unit for sensing pressure when touching the touchscreen to generate writing pressure information and a signal processor for modulating the pen driving signal in response to the signal level of the writing pressure information and outputs the modulated signal as a pen driving signal in which the writing pressure information has been reflected.