Content abstract: There are three main genres of polyphonic music works formed in the history of music: innovative music, fugue and cannon. "Two Inventions" is part of Bach's etudes for his youngest son William F. Lidman Bach during 1720- 1722 (namely "Keyboard Tone for wilhelm friedemann bach"). It was named "prelude" and later changed to "invention". "Two Composed Songs" is a short theme as the material, using imitation, cannon, counterpoint and other methods to change and develop, creating a new ditty. Most of them are composed of three parts, and a few are two parts (such as 10, 12, etc. ). As Bach pointed out in the preface of 1723 Manuscript, these works are used in teaching, which enables "lovers and learners of classical piano not only to play two parts beautifully, but also to play three parts correctly and skillfully after mastering them." . At the same time, it also puts forward clear methods for obtaining the development skills of works creation, understanding the playing methods of songs and generating a strong desire to compose music. "Today, we still regard Two Composed Songs as a bridge to polyphonic masterpieces. Learning "Two Composed Songs" is a necessary and important basis for learning polyphonic works with three or more voices in the future. From this, we can not only understand the style and interest of Bach's works, but also improve the ability of controlling the keyboard independently with both hands, train keen musical hearing, strengthen the logical thinking of polyphonic music and train different methods of touching keys with fingers. Historically, Beethoven, Chopin, Mendelssohn and other masters all attached great importance to the study, research and performance of Bach's music.