Abstract: A novel method for automatic music transcription based on harmonic structure information is developed in this paper. The instruments of the same class have similar timber and harmonic structure. Therefore, harmonic structure information of an instrument is acquired beforehand and is used to construct the loudness spectrum for the unknown instrument of the same class in music transcription. The inharmonicity factor and the error in note fundamental frequency are considered during the spectrum construction. The loudness spectrum of input audio can be considered approximately as the linear sum of spectra corresponding to the mixed notes and the truncated total least squares is used to solve the sum equation. Experimental results of piano music transcription show that the method not only can transcribe music of an unknown instrument with high performance, but also can distinguish the loudness of mixed notes.