We present a novel technique to measure the polarization of hadronic decays of boosted W bosons using jet substructure observables. A new proxy variable, \(p_\theta \) , is introduced, which is constructed purely from subjet energies to estimate the parton-level decay polar angle, \(\theta _*\) of the W boson in its rest frame. The distribution of this variable is sensitive to W boson polarization and provides a process-independent method for measuring longitudinal and transverse polarization fractions. We validate our approach by applying the proposed technique to vector boson scattering (VBS) at the High-Luminosity Large Hadron Collider (HL-LHC) and demonstrate its viability in measuring hadronic W polarization.