  A wealth of Galactic accreting X-ray binaries have been observed both in
low/hard state and high/soft state. The transition between these two states was
often detected. Observation shows that the transition luminosity between these
two states is different for different sources, ranging from 1% to 4% of the
Eddington luminosity. Even for the same source the transition luminosity at
different outbursts is also different. The transition can occur from 0.0069 to
0.15 Eddington luminosity. To investigate the underlying physics, we study the
influence of viscosity parameter $\alpha$ on the transition luminosity on the
basis of the disk-corona model for black holes. We calculate the mass
evaporation rate for a wide range of viscosity parameter, $0.1\le \alpha\le
0.9$. By fitting the numerical results, we obtain fitting formulae for both the
transition accretion rate and the corresponding radius as a function of
$\alpha$. We find that the transition luminosity is very sensitive to the value
of $\alpha$, $L/L_{\rm Edd}\propto\alpha^{2.34}$. For $0.1\le\alpha\le 0.6$,
the transition luminosity varies by two orders of magnitude, from 0.001 to 0.2
Eddington luminosity. Comparing with observations we find that the transition
luminosity can be fitted by adjusting the value of $\alpha$, and the model
determined values of $\alpha$ are mostly in the range of observationally
inferred value. Meanwhile we investigate the truncation of the disk in the
low/hard state for some luminous sources. Our results are roughly in agreement
with the observations.
