  Since synchrotron polarization fluctuations are related to the fundamental
properties of the magnetic field, we propose the polarization intensity
variance to measure the Galactic interstellar medium (ISM) magnetization. We
confirm the method's applicability by comparing it with the polarization angle
dispersion and its reliability by measuring the underlying Alfv\'enic Mach
number of MHD turbulence. With the finding of the power-law relation of
$\mathcal{A} \propto M_{\rm A}^{2}$ between polarization intensity variance
$\mathcal{A}$ and Alfv\'enic Mach number $M_{\rm A}$, we apply the new
technique to the Canadian Galactic Plane Survey (CGPS) data, achieving
Alfv\'enic Mach number of the Galactic ISM. Our results show that the
low-latitude Galactic ISM is dominated by sub-Alf\'enic turbulence, with
$M_{\rm A}$ approximately between 0.5 and 1.0.
