  In this paper we study an impurity in Bose-Einstein-Condensate system at
$T=0K$ and suppose the contact forms for boson-boson and boson-impurity
interactions. Using Bogoliubov theory and a further approximation corresponding
to only think over the forward scattering of impurity by bosons, we derive a
reduced Hamiltonian whose form is the same as the Fr\"{o}hlich Hamiltonian for
large polaron. By using Lee-Low-Pines (LLP) theory for large polaron, we obtain
the effective mass of impurity, the phonon number carried by impurity and the
energy related to the existence of impurity. In addition, we also discuss the
valid range for forward-scattering approximation.
