With the promotion of “Internet +”, computer-assisted teaching has been developed rapidly. Due to the particularity of piano teaching, piano teaching is still in a relatively traditional mode. Currently, how to apply Internet technology to piano teaching is the focus of research on piano education. This paper proposed a design principle for piano teaching system based on augmented reality technology, designed a piano teaching system under the guidance of such principle and applied this system in actual piano teaching. The students’ feedbacks have been collected to help enhancing the effects of the designed system