A touch input system includes a touch panel, configured to transmit an uplink signal; and an active stylus, configured to analyze the uplink signal, synchronize timing and bi-directionally communicate with the touch panel according to the uplink signal; wherein the uplink signal includes a preamble, for synchronizing the timing; a digital data, for bi-directionally communicating between the active stylus and the touch panel; and a cyclic redundancy check, for executing an error check and an error correction for data.