The present invention provides a method for detecting a transmitter approximating or touching a touch sensitive device. The transmitter transmits an electrical signal mixed by signals having a plurality of frequencies. The touch sensitive device includes a plurality of first electrodes, a plurality of second electrodes and a plurality of sensing points intersected by those first and second electrodes. The method includes: calculating the signal strength of the electrical signal received by each of the first electrodes and second electrodes; and calculating a relative position between the transmitter and the touch sensitive device according to the calculated signal strengths.