The Trinity is the most important mystery of Christian philosophy. To understand it, there have developed two different, even opposed, approaches to it. One is Social Trinitarianism, starting from the three distinctive Persons and then talking about the unity of them, represented by the Greek Fathers; the other one is Augustine's psychological Trinitarian model, starring from the unity of God and then talking about its different Persons. The latter has deeply influenced the whole of western Trinitarianism. In this article, the author first point: out the essential problem of Trinity, then discusses Augustine's psychological analogies in detail, noticing his thought's complexity, and, at last, he introduces the Social Trinity tradition and its modern representives' criticism of the so called Plotinus- Augustine - Thomas Aquina-Karl Barth Route.