Leonard Bernstein（1918–1990）was a American conductor international renowned, a composer of diversification works, and a musical educator through television lectures on classical music. All this various development marks Bernstein as an unusually versatile figure. His musical personal development and innovative thought form the magnificent achievement. Bernstein has a representative position in American music of the twentieth century. West Side Story is a significant successfully musical, also an effective introduction to Bernstein’s output. There are numbers of his common stylistic traits in West Side Story, such as dramatic contrast, jazz elements, motivic development. The work includes both art music and jazz element. By a combination of art music and jazz element, he made a appealing music style through wide-ranging eclecticism in all of his works, especially West Side Story. This study focused on analysis of theatricality and eclecticism in West Side Story. Also make connection between Bernstein’s musical thoughts and works, presenting his unique creation and the contribution in American music.