We have investigated the quasiparticle dynamics and collective excitations in the quasi-one-dimensional material ZrTe$_5$ using ultrafast optical pump-probe spectroscopy. Our time-domain results reveal two coherent oscillations having extremely low energies of $hbaromega_1sim$0.33 meV (0.08 THz) and $hbaromega_2sim$1.9 meV (0.45 THz), which are softened as the temperature approaches two different critical temperatures ($sim$54 K and $sim$135 K). We attribute these two collective excitations to the amplitude mode of charge density wave instabilities in ZrTe$_5$ with tremendously small nesting wave vectors. Furthermore, scattering with the $hbaromega_2$ mode may result in a peculiar quasiparticle decay process with a timescale of $sim$1-2 ps below the transition temperature $T^*$ ($sim$135 K). Our findings provide pivotal information for studying the fluctuating order parameters and their associated quasiparticle dynamics in various low-dimensional topological systems and other materials.
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