  Mrk 501 is a prototypical high-synchrotron-peaked blazar (HBL) and serves as
one of the primary targets for the {\it Imaging X-ray Polarimetry Explorer}
({\it IXPE}). In this study, we report X-ray polarization measurements of Mrk
501 based on six {\it IXPE} observations. The detection of X-ray polarization
at a confidence level exceeding 99\% is achieved in four out of the six
observations conducted across the entire energy range (2--8 keV) of {\it IXPE}.
The maximum polarization degree ($\Pi_{\rm X}$) is measured to be
$15.8\%\pm2.8\%$, accompanied by a polarization angle ($\psi_{\rm X}$) of
$98.0\deg\pm5.1\deg$ at a confidence level of $5.6 \sigma$. During the
remaining two observations, only an upper limit of $\Pi_{\rm X}<$12\% could be
derived at the 99\% confidence level. No temporal variability in polarization
is observed throughout all six {\it IXPE} observations for Mrk 501. A
discernible trend of energy-dependent variation in the polarization degree is
detected in optical spectropolarimetry; however, no analogous indication is
observed in $\Pi_{\rm X}$. The chromatic behavior of $\Pi$ and the consistent
values of $\psi$ across different frequencies from X-rays to radio waves, along
with the agreement between $\psi$ and jet position angle, strongly support the
interpretation of the energy-stratified model with shock-accelerated particles
in the jet of Mrk 501. Additionally, the possibility of the presence of a
global helical magnetic field in the jet of Mrk 501 is discussed.
