A relativistic electron-positron ($e^{+}e^{-}$) pair wind from a rapidly rotating, strongly magnetized neutron star (NS) would interact with a gamma-ray burst (GRB) external shock and reshapes afterglow emission signatures. Assuming that the merger remnant of GW170817 is a long-lived NS, we show that a relativistic $e^{+}e^{-}$ pair wind model with a simple top-hat jet viewed off-axis can reproduce multi-wavelength afterglow lightcurves and superluminal motion of GRB 170817A. The Markov chain Monte Carlo (MCMC) method is adopted to obtain the best-fitting parameters, which give the jet half-opening angle $theta_{j}approx0.11$ rad, and the viewing angle $theta_{v}approx0.23$ rad. The best-fitting value of $theta_{v}$ is close to the lower limit of the prior which is chosen based on the gravitational-wave and electromagnetic observations. In addition, we also derive the initial Lorentz factor $Gamma_{0}approx47$ and the isotropic kinetic energy $E_{rm K,iso}approx2times10^{52}rm erg$. A consistence between the corrected on-axis values for GRB 170817A and typical values observed for short GRBs indicates that our model can also reproduce the prompt emission of GRB 170817A. An NS with a magnetic field strength $B_{p}approx1.6times10^{13}rm G$ is obtained in our fitting, indicating that a relatively low thermalization efficiency $etalesssim10^{-3}$ is needed to satisfy observational constraints on the kilonova. Furthermore, our model is able to reproduce a late-time shallow decay in the X-ray lightcurve and predicts that the X-ray and radio flux will continue to decline in the coming years.
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