  It is shown that explicit B\"{a}cklund transformations (BTs) for the
high-order constrained flows of soliton hierarchy can be constructed via their
Darboux transformations and Lax representation, and these BTs are canonical
transformations including B\"{a}cklund parameter $\eta$ and possess a
spectrality property with respect to $\eta$ and the 'conjugated' variable $\mu$
for which the pair $(\eta, \mu)$ lies on the spectral curve. As model we
present the canonical explicit BTs with the spectrality for high-order
constrained flows of the Kaup-Newell hierarchy and the KdV hierarchy.
