Granular flow is usually divided into three kinds of flow pattern, namely quasi static flow, slow flow, and rapid flow. The core issue of the research is the constitutive relation. A series of constitutive relations of application value have been received up to now, however, the study on principal theory is insufficient. Granular flow has an emergent mesoscopic structure, such as force chain network and vortex, involving complex irreversible processes. This paper studies its mesoscopic structure and principal characters, introduces the concept of two granular temperatures Tconf and Tkin of the granular flow to characterize the degree of chaotic motion and disordered configuration evolution, sets them as the non-equilibrium variables to constitute the thermodynamic state variables set for granular flow with the classical irreversible thermodynamic (CIT) variables, also determines the granular flow law of energy conversion and the entropy production rate, etc., and develops the two granular temperatures (TGT) model. Taking the simple shear quasi-static granular flow in a constant volume as example, and combining it with the discrete element method (DEM), this work confirms the material parameters needed for the TGT model, and analyzes the law of developing period and the effective coefficient of friction of steady period of granular flow.