Due to the polyanionic nature of RNAs, the structural folding of RNAs are sensitive to solution salt conditions, while there is still lack of a deep understanding of salt effect on the thermodynamics and kinetics of RNAs at a single base pair level. In this work, the thermodynamic and the kinetic parameters for the base pair AU closing/opening at different salt concentrations were calculated by 3-μs all-atom molecular dynamics (MD) simulations at different temperatures. It was found that for the base pair formation, the enthalpy change ∆H is nearly independent of salt concentration, while the entropy change ∆S exhibits a linear dependence on the logarithm of salt concentration, verifying the empirical assumption based on thermodynamic experiments. Our analyses revealed that such salt concentration dependence of the entropy change mainly results from the dependence of ion translational entropy change for the base pair closing/opening on salt concentration. Furthermore, the closing rate increases with the increasing of salt concentration, while the opening rate is nearly independent of salt concentration. Additionally, our analyses revealed that the free energy surface for describing the base-pair opening and closing dynamics becomes more rugged with the decrease of salt concentration.