  In this paper, the conventional Holstein-Primakoff method is generalized with
the help of the characteristic angle transformation [Lei Zhou and Ruibao Tao,
J. Phys. A {\bf 27} 5599 (1994)] for the spin-one magnetic systems with
single-ion anisotropies. We find that the weakness of the conventional method
for such systems can be overcome by the new approach. Two models will be
discussed to illuminate the main idea, which are the ``easy-plane" and the
``easy-axis" spin-one ferromagnet, respectively. Comparisons show that the
current approach can give reasonable ground state properties for the magnetic
system with ``easy-plane" anisotropy though the conventional method never can,
and can give a better representation than the conventional one for the magnetic
system with ``easy-axis" anisotropy though the latter is usually believed to be
a good approximation in such case. Study of the easy-plane model shows that
there is a phase transition induced by the external field, and the
low-temperature specific heat may have a peak as the field reaches the critical
value.
