Facing the urgent needs of piano teaching informatization construction in the post-epidemic era, a research on optical teaching instrument suitable for remote online guidance on piano practice is carried out. According to the teacher's active correction on students' hand shape and fingering in the process of piano teaching, an optical instrument design method for projection and imaging integrated light path design is proposed to meet needs of the online visual analysis and optical guidance of the teacher in the process of individual piano fingering exercises which is usually carried out offline. By adopting the spherical off-axis four-inverted optical structure, the teaching instrument realizes the compact design of the large field of view and wide spectrum optical system, and the optical system envelope is only 110mm×35m m×30mm. The design results show that the teaching instrument can realize the projection and imaging of the surface within the range of 1240mm of line length at a height of 500mm from the keyboard (the top of the piano score bracket), which can fully cover the imaging requirements of the whole piano keyboard (1200mm); The imaging spectrum of the optical system covers 450nm~900nm, and the MTF of the full field of view reaches 0.6 (@33lp/mm) or more, which can fully meet the requirements of the digital and symbolic optical projection of any piano key as well as the clear imaging of the fingers, and thus realizes the effect of offline teaching in the process of online piano teaching. spherical off-axis four-reflective optical system, projection and imaging integrated light path, optical teaching instrument