  In this paper, we reduce pointwise convergence of polynomial ergodic averages
of general measure-preserving system acted by $\mathbb{Z}^{d}$ to the case of
measure-preserving system acted by $\mathbb{Z}^{d}$ with zero entropy. As an
application, we can build pointwise convergence of polynomial ergodic averages
for $K$-system acted by $\mathbb{Z}^{d}$.
