  We propose a scheme for preparation of large-scale entangled $W$ states based
on the fusion mechanism via quantum Zeno dynamics. By sending two atoms
belonging to an $n$-atom $W$ state and an $m$-atom $W$ state, respectively,
into a vacuum cavity (or two separate cavities), we may obtain a ($n+m-2$)-atom
$W$ state via detecting the two-atom state after interaction. The present
scheme is robust against both spontaneous emission of atoms and decay of
cavity, and the feasibility analysis indicates that it can also be realized in
experiment.
