Abstract: The effects and its explanation in the point of view of molecular physics are discussed in this paper Accumulative and macroscopic features of universal gravitation are determined by its' monopole characteristics and it has almost no effects on free molecules. However, inside liquid, Especially in the liquor, obvious effects from gravity on molecular groups exist in the form of macroscopically static pressure, as gravity gets smaller and smaller, the static pressure inside liquid will disappear gradually and the secondary phenomena, such as sedimentation and floating in different phases and floating convection in same phases will also disappear. In this paper, it is pointed out that Bossinesq approximation is substantially a hypothesis of linear thermodynamics or quasi-equilibrium state, so it is possible to express the nondimensional numbers in the critical point with some physical property parameters in the initial equilibrium state. An idea is put forward that by eliminating gravity to induce a transform from non-equilibrium state to equilibrium state. The expression of Rayleigh number Ra derived in this paper shows that as gravity acceleration disappearing, the value of Ra will change from a turbulent state far from equilibrium to a ray Leigh-Benard convection cell in a stable flow, even to an equilibrium state without macroscopic flow. Finally, this theoretical explanation model is applied to estimate the microgravity level necessary for space materials processing and the interchangeability of low vacuum and microgravity in simulation of the heat conduction state in natural convections of gases in space devices.