The debate between explanation and understanding originated from the debate between Dilthey and Ebbinghaus, and run though the development of psychology. It seems that this is a debate about the purpose and task of psychology. In fact, it is a debate between two kinds of psychology with different forms and routes. The task of explanation psychology is to explain the nature, the rule and the mechanism of the mental and behavior. There are four forms of explanation psychology, such as mental-mental and mental-object relation explanation, stimulate-reaction explanation, cognitive mental-neural mechanisms explanation, evolved psychological mechanism explanation. The methodology of explanation psychology has the distinctive feature of scientism, which emphasizes physicalism, mechanism, biologism worldview, correspondence theory of epistemology, and reductionism, objectivism, logicism methodology. There are different research topics during every period of the development of explanation psychology. Therefore, it focuses on different key points. Every period of the development of explanation psychology has different features of scientism. The methodology of the creation period of explanation psychology can be called psychologism paradigm. The methodology of behavioral explanation psychology can be called behaviorism paradigm. The methodology of modern cognitive psychology can be called information processing paradigm. Both cognitive neuroscience and evolutionary psychology expanded the scientism ideological line of psychology. During the process of the application of the theory and method of natural science, it contributed to forming a more solid foundation of natural science for the scientism. During the history of the psychology development, the explanation psychology and the understanding psychology hold opposite ideas and strongly disagree with each other. But it is clear that the explanation psychology has always led the mainstream of psychology. On one side, the explanation psychology has been devoted to the study of human nature, highlighted the scientism pursuit of psychology, built the natural image of human. Therefore it has expended the connotation of psychology as a natural science. While the understanding psychology is in a non mainstream position. On the other side, it raised the social and cultural nature of human, showed the humanistic care of psychology, built the image of human which is different from object, and drew the profile of psychology as a humanistic science. Both the explanation psychology and the understanding psychology has promoted the independence and maturity of psychology, and both of them contribute to the reality of today's psychology. In the future, in order to build a complete and clear image of human, in order to show the overall nature of human as the ideal, we could predict that the healthy development of psychology needs to abandon the either-this-or-that way of thinking that embedded in both explanation psychology and understanding psychology. Therefore the establishment of an integrated view of psychology is badly needed. Only in this way, many opposite poles in both explanation psychology and understanding psychology, such as explanation and understanding, reductionism and holism, objectivism and subjectivism, logicism and intuitionism, etc, can find their proper space, psychology will achieve a complete unity of thorough scientism and humanism, and only in this way psychology in the future will be able to burden the responsibility to build the complete image and show the full nature of human.