The prevalence of expressionism in the early twenty century was strongly related to the social environment at that time. Under the political situation of militarism and absolute monarchy, the society was facing a severe turbulence in Germany. And, an economic system based on capitalism also lead to extreme disparity between the rich and the poor. Additionally, the scientific study of rationality revealed the importance of caring for the inside needs, which gradually resulted in the prosperity of expressionism in different kinds of arts fields, including literature, theater, painting, music and so on. Expressionists started to express their feeling in a distorted way in order to show their true self and emotional experience. Arnold Schoenberg, who was one of the chief representatives in music of expressionism, used atonal composition skill as a reflection of his dissatisfaction against the reality. This study is divided into three main parts, excluding the part of introduction and conclusion. In the first part, the origin and style of expressionism in the early twenty century is introduced. Secondly, factors leading to the transformation of Schoenberg¡¦s composition skill, from the conventional method to the innovative atonal technique, are further discussed, which include the effect from other expressionists in other fields and his living environment. In the last part, Drei Klavierstucke, Opus 11 is taken as an example to demonstrate its historical background, music features, and composition techniques used in the work to specifically tell the content of expressionism.