In conventional thermodynamics, energy is regarded as a physical quantity transferring from one system to another, but in present study, the real energy is regarded as a physical quantity coming out from one interaction and absorbing into another interaction between two systems. To reconstruct thermodynamics with such a point of view, available work is distinguished from half work in conventional work concepts, and a special space named reversible world is proposed in which every process is reversible and the only measurable quantity is available work and just the equality between the intensities of two systems can be verified. As results, thermodynamic laws are arranged into two principles in the reversible world-conservations of energy elements and conservation of available energy. It means the exsistences of state properties corresponding to transferring energy elements and the available work. The former are extensive properties and the later is named potential work which is a property of the composite system and a kind of mathematical distance. The conventional available energy (exergy) and internal energy can be explained as the special cases of potential work, and the conventional first law of thermodynamics can be derived from the principle of the conservation of available energy. With these new concepts, the description of thermodynamic processes is more comprehensive. The second law of thermodynamics is no longer needed in the reversible world.