The state transitions of black hole (BH) X-ray binaries 
are discussed based on the
evolution of large-scale magnetic fields, in which the 
combination of three energy
mechanisms are involved: (1) the Blandford-Znajek (BZ) 
process related to the open
field lines connecting a rotating BH with remote 
astrophysical loads, (2) the
magnetic coupling (MC) process related to the closed 
field lines connecting the BH
with its surrounding accretion disk, and (3) the 
Blandford-Payne (BP) process related
to the open field lines connecting the disk with remote 
astrophysical loads. It turns
out that each spectrum state of the BH binaries 
corresponds to each configuration of
magnetic field in BH magnetosphere, and the main 
characteristics of low/hard (LH)
state, hard intermediate (HIM) state, steep power law 
(SPL) state and
thermal–dominant (TD) state are roughly fitted based 
on the evolution of large-scale
magnetic fields associated with disk accretion. Two 
kinds of the magnetic
instabilities are invoked to interpret the transitions from 
LH to SPL states and from
SPL to TD states.