  We study the dynamical spin excitations of the one-dimensional $S=1$
Heisenberg antiferromagnetic chain with single-ion anisotropy by using quantum
Monte Carlo simulations and stochastic analytic continuation of imaginary-time
correlation function. Using the transverse dynamic spin structure factor, we
observe the quantum phase transition with a critical point between the
topological Haldane gapped phase and the trivial phase. At the quantum critical
point, we find a broad continuum characterized by the Tomonaga-Luttinger liquid
similar to a $S=1/2$ Heisenberg antiferromagnetic chain. We further identify
that the elementary excitations are fractionalized spinons.
