  The self-consistent mean field approximation of two-flavor NJL model with
introducing a free parameter $\alpha$ to reflect the competition between
"direct" channel and the "exchange" channel, is employed to study QCD phase
structure at finite isospin chemical potential $\mu_I$, finite baryon chemical
potential $\mu_B$ and finite temperature $T$, especially the location of the
QCD critical point. It is found that, for fixed isospin chemical potentials the
lower temperature of phase transition is obtained with $\alpha$ increasing in
the $T-\mu_I$ plane, and the largest difference of the phase transition
temperature with different $\alpha$'s appears at $\mu_I \sim 1.5m_{\pi}$. At
$\mu_I=0$ the temperature of the QCD critical end point (CEP) decreases with
$\alpha$ increasing, while the critical baryon chemical potential increases. At
high isospin chemical potential ($\mu_I=500$ MeV), the temperature of the QCD
tricritical point (TCP) increases with $\alpha$ increasing, and in the regions
of low temperature the system will transit from pion superfluidity phase to the
normal phase as $\mu_B$ increases. At low temperatures, the critical
temperature of QCD phase transition with different $\alpha$'s rapidly increases
with $\mu_I$ at the beginning, and then increases smoothly around $\mu_I>300$
MeV. In high baryon density region, the increase of the isospin chemical
potential will raise the critical baryon chemical potential of phase
transition.
