  Quintessence field is a widely-studied candidate of dark energy. There is
"tracker solution" in quintessence models, in which evolution of the field
$\phi$ at present times is not sensitive to its initial conditions. When the
energy density of dark energy is neglectable ($\Omega_\phi\ll1$), evolution of
the tracker solution can be well analysed from "tracker equation". In this
paper, we try to study evolution of the quintessence field from "full tracker
equation", which is valid for all spans of $\Omega_\phi$. We get stable fixed
points of $w_\phi$ and $\Omega_\phi$ (noted as $\hat w_\phi$ and
$\hat\Omega_\phi$) from the "full tracker equation", i.e., $w_\phi$ and
$\Omega_\phi$ will always approach $\hat w_\phi$ and $\hat\Omega_\phi$
respectively. Since $\hat w_\phi$ and $\hat\Omega_\phi$ are analytic functions
of $\phi$, analytic relation of $\hat w_\phi\sim\hat\Omega_\phi$ can be
obtained, which is a good approximation for the $w_\phi\sim\Omega_\phi$
relation and can be obtained for the most type of quintessence potentials. By
using this approximation, we find that inequalities $\hat w_\phi<w_\phi$ and
$\hat\Omega_\phi<\Omega_\phi$ are statisfied if the $w_\phi$ (or $\hat w_\phi$)
is decreasing with time. In this way, the potential $U(\phi)$ can be
constrained directly from observations, by no need of solving the equations of
motion numerically.
