  The disk-corona evaporation model naturally interprets many observational
phenomena in black hole X-ray binaries, such as the truncation of an accretion
disk and the spectral state transitions. On the other hand, magnetic field is
known to play an important role in transporting angular momentum and producing
viscosity in accretion flows. In this work, we explicitly take the magnetic
field in the accretion disk corona into account and numerically calculate the
coronal structure on the basis of our two-temperature evaporation code. We show
that the magnetic field influences the coronal structure by its contribution to
the pressure, energy and radiative cooling in the corona and by decreasing the
vertical heat conduction. We found that the maximal evaporation rate keeps more
or less constant ($\sim 0.03$ Eddington rate) while the strength of magnetic
fields changes, but that the radius corresponding to the maximal evaporation
rate decreases with increasing magnetic field. This predicts that the spectral
state transition always occurs at a few percent of Eddington accretion rate,
while the inner edge of thin disk can be at $\sim 100 R_{\rm S} $ or even less
in the hard state before the transition to the soft state. These results
alleviate the problem that previous evaporation models predict too large a
truncation radius, and are in better agreement with the observational results
of several black hole X-ray binaries, though discrepancies remain.
