  We explore the quasiparticle dynamics in bilayer nickelate La$_3$Ni$_2$O$_7$
crystal using ultrafast optical pump-probe spectroscopy at high pressure up to
34.2 GPa. At ambient pressure, the temperature dependence of relaxation
indicates appearance of phonon bottleneck effect due to the opening of
density-wave-like gap at 151 K. By analyzing the data with RT model, we
identified the energy scale of the gap to be 70 meV at ambient pressure. The
relaxation bottleneck effect is suppressed gradually by the pressure and
disappears around 26 GPa. At high pressure above 29.4 GPa, we discover a new
density-wave like order with transition temperature of $\sim$130 K. Our results
not only provide the first experimental evidence of the density-wave like gap
evolution under high pressure, but also offering insight into the underline
interplay between the density wave order and superconductivity in pressured
La$_3$Ni$_2$O$_7$.
