The present invention provides a touch sensitive processing apparatus for receiving electrical signals carrying pressure information transmitted from a first stylus, comprising: a sensing circuit, configured for receiving the electrical signals via electrodes of a touch panel; and a processor, coupled to the sensing circuit, configured for: despreading a first preamble code of the received electrical signals in accordance with a pseudo-random number (PN) code in a first time period; despreading a second preamble code of the received electrical signals in accordance with the PN code in a second time period; and calculating the pressure information according to a first signal strength ratio of a first part of the received electrical signal and a second part of the received electrical signal, wherein the first part comprises the first preamble code and the second part comprises the second preamble code.