A music composition method and system is provided for composing a music piece using a touch interaction with a touchscreen-enabled portable device. A music composition method includes setting an accompaniment with chords varying according to the progress of a unit play time; playing the accompaniment; displaying, when a series of touches are detected, a trace of the touches; playing a melody of tones mapped to positions of the touches and matching notes of a note scale defined by chords of the accompaniment playing at the times when the touches are detected; and saving the trace, melody, and accompaniment as a music piece as an ensemble.