The foreword of this paper is to introduce how the literary and philosophical mood in Paris after World War Ⅰ was conducive to the persistent development of existential philosophy in France. While the famous French scholars were absorbing the scientific thoughts from Germany, Ecole Normale Super Paris also produced some young instructors of philosophy, who transformed existential philosophy into Sartre's existentialism after surrealism started to decline. The second part of the paper is devoted to the development of existential philosophy, from traditional existential philosophy to the two branches of existentialist thoughts: the atheistic existentialism and the Christian atheism. Between the two world wars, Sartre carried on the atheistic existentialism, launching Les Temps Modernes as his mouthpiece, as Albert Camus joined the editorial board of Combat to address the absurdity of human conditions. The two of them piloted the movement of existential literature, interpreting the absurdity of life in the literary forms of novels and dramas. As World War Ⅱ confronted human beings with the cruelty of the transience of life, Sartre's lecture, ”Existentialism is a Humanism,” advocated his Sartrism that was informed of the atheistic slogan, ”Existence precedes essence.” The third part is about some young people who followed the theme of Camus”s The Myth of Sisyphus, the ”absurdity of being.” Modeling after Samuel Beckett and Ionesco, these young writers are very active in the theatre of France, creating one after another work of the new genre that has made profound impact on western drama: theatre of the absurd.