Lord of the Flies, a fictional novel, tells the story of a plane shot down and a group of boys were abandoned on a deserted coral island in a future nuclear war. At first, the children enjoyed a comfortable life without adult supervision and made joint efforts to face the difficulties. But happy times didn't last long, for fearing the so-called "wild animals", they split into two parties, and ended up with the instinctive despotism overwhelming the rational democracy. This article intends to use Freud's personality structure theory to analyze the main characters in Lord of the Flies. Through the analysis of Jack and Roger's increasingly stronger id, Ralph and Piggy's weak ego and Simon's disappearance of superego, it reflects the continuous development and change of human nature's good and evil in the novel. It reveals that with the weakness of civilization and democracy, the evil side of human society is in a dominant position. It is hoped that human beings can seriously reflect on the evil side of their own human nature and find a way to redeem the neglected good.