  We study the $CP$ asymmetry of $B^\pm\to \omega K^\pm$ with the inclusion of
the $\rho-\omega$ mixing mechanism. It is shown that the $CP$ asymmetry of
$B^\pm\to\omega K^\pm$ experimentally measured ($A_{CP}^{\text{exp}}$) and
conventionally defined ($A_{CP}^{\text{con}}$) are in fact different, which
relation can be illustrated as $A_{CP}^{\text{exp}}=A_{CP}^{\text{con}}+\Delta
A_{CP}^{\rho\omega}$, with $\Delta A_{CP}^{\rho\omega}$ the $\rho-\omega$
mixing contribution to $A_{CP}^{\text{exp}}$. $A_{CP}^{\text{exp}}$ is in fact
the regional $CP$ asymmetry of $B^\pm\to\pi^+\pi^-\pi^0 K^\pm$ when the
invariant mass of the three pions lies in the vicinity of the $\omega$
resonance. The numerical value of $\Delta A_{CP}^{\rho\omega}$ is extracted
from the experimental data of $B^\pm\to\pi^+\pi^-K^\pm$ and is found to be
comparable with $A_{CP}^{\text{exp}}$, hence, nonnegligible. The conventionally
defined $CP$ asymmetry, $A_{CP}^{\text{con}}$, is obtained from the values of
$A_{CP}^{\text{exp}}$ and $\Delta A_{CP}^{\rho\omega}$, and is compared with
the theoretical calculations in the literature.
