Tradition geoscience, which is established in foundation of studies on solid part of Earth, has not placed the Earth's interior fluids in an important position, except its beginning—sedimentary geology and its end—metallogenetic geology. Explanation methods and integral answers for many important questions, such as transformation, exchange of Earth's substance and energy, can't be found in its knowledge system. Developping modern geoscience have been thinking of Earth's interior fluids as main study object, and playing it in all fields of geoscience. The basic points of modern geoscience may be as fellow: fluids exist widely and move incessantly in all round of the Earth, it is as important as solid part of the Earth, and must be accepted as the main study object of modern geoscience. The Earth's interior fluids not only play an important role in various geological functions, but also can be viewed as a primary reason of all geological functions, that is to say, it penetrate from beginning to ending of all geological functions (such as tectonic movement, magmatic activity, metamorphism, sedimentation, mineralization and geological calamity). All geological functions occured in Earth's interior are united organically by the Earth's interior fluid. Accoding to the relationships between fluid and special geological function, Earth's interiour fluid can be devided into three types with the natures of obtainning-circularing: beginning fluid, process fluid and ending fluid. The role of beginning fluid in a geological event are discussed and a new basic concept of united geological science of Earth's interior fluid is put forward in this paper.