  Using the valence-bond-solid (VBS) approach and the Schwinger boson mean
field approximation, we study the dependence of the Haldane gap of a spin-1
linear chain Heisenberg antiferromagnet on impurity doping with different
spins. The impurity spins affect the singlet pairing order parameter $\Delta $
and the constraint factor $\lambda$. As a result, the Haldane gap is reduced by
a factor $ \sim n_i^{2/3}$, with $n_i$ as the impurity concentration, and
eventually collapses at $n_i \sim 1/\xi$ with $\xi$ as the VBS correlation
length. This theoretical prediction can be verified by neutron scattering
experiments.
