  In order to achieve a deeper understanding of gravity theories, it is
important to further investigate the thermodynamic properties of black hole at
the critical point, besides the phase transition and critical behaviors. In
this paper, by using Maxwell's equal area law, we choose $T,Q,\Phi$ as the
state parameters and study the phase equilibrium problem of general
$(n+1)$-dimensional RN-AdS black holes thermodynamic system. The boundary of
the two-phase coexistence region and its isotherm and isopotential lines are
presented, which may provide theoretical foundation for studying the phase
transition and phase structure of black hole systems.
