  In this paper, using Maxwell's equal-area law we study the phase transition
of charged AdS black holes by choosing different independent conjugate
variables. As is well known, the phase transition can be characterized by the
state function of the system, the determination of the phase transition point
has nothing to do with the choice of independent conjugate variables. To
studying the thermodynamic properties of AdS black holes we give the conditions
under which the independent conjugate variables are chosen. When the charge of
the black hole is invariable, according to the conditions we find that the
phase transition is related to the electric potential and the horizon radius of
the charged black hole. Keeping the cosmological constant as a fixed parameter,
the phase transition of a charged AdS black hole is related to the ratio of the
event horizon to cosmological constant of the black hole. This conclusion is of
great importance for us to study the critical phenomenon of black holes and to
improve the self-consistent geometry theory of black hole thermodynamic.
