A method for providing a 3D audio work includes providing a one-ear HRTF filter and a related function synthesizer storing a related function therein, and inputting sound signals into the one-ear HRTF filter. The sound signals are converted into one-ear output sound signals which are received by one ear and synthesized to output sound signals for the other ear. A method for providing the related function includes inputting sound signals into HRTF filters of opposite ears and obtaining output sound signals which respectively act as raw signals and target signals. The raw signals are synthesized by a synthesizer to output sound signals which compare with the target signals. A related function registered in the synthesizer is accordingly regulated so as to obtain the related function which satisfies a minimum difference between the output sound signals from the synthesizer and the target signals.