  The comprehensive study of the temperature dependent x-ray absorption
spectroscopy (XAS) reveals a dynamical spectral weight $\alpha$ in
YBa$_2$Cu$_3$O$_y$ (YBCO). Large spectral weight changes for both the Upper
Hubbard band and the Zhang-Rice band due to dynamics of holes are
experimentally found in the underdoped regime. A large value of $\alpha \geq
0.3$ is indispensable to describing XAS of YBCO with the conservation of
states. The value of $\alpha$ is linearly proportional to the pseudogap
temperature in the underdoped regime, but becomes smaller as the doping level
goes to the undoped limit. Our results clearly indicate that the pseudogap is
related to the double occupancy and originates from bands in higher energies.
