To examine the ablation dynamics of silver thin films by femtosecond laser, we experimentally investigate the plume evolution and behavior of ejected nanoparticles (NPs) via emission and scattering spectroscopy measurements under background pressures of 760 torr, 5 torr, 5 x 10-3 torr, to 3 x 10-5 torr. The emission spectroscopy experiments show that the propagation of the ablated plume is affected by ambient pressure. The higher the pressure, the more the propagation of the plasma is suppressed. Under higher vacuum, the lifetime of plasma is shorter due to diminished collisions with background molecules. The evolution of plasma lasts more than 200 ns under 760 torr while it does not exceed 200 ns under high vacuum (3 x 10-5 torr). Through the scattering measurements, the average propagation speed of NPs is 200 m/s under 3 x 10-5 torr, 190 m/s under 5 x 10-3 torr, 155 m/s under 5 torr, and 120 m/s under 760 torr. The ejected nanoparticles from the periphery of the ablated spot exhibit oblique trajectories because of the exerted recoil pressure at the spot center region that is subject to high incident energy densities.