The problem of the coupling system of carrier and ferromagnetic material has been transformed into that of the coupling system of carrier and two kinds of Bosons by using Schwinger's oscillator theory of angular momentum. That the existence of bound polaron in ferromagnetic material depends on the existence of impurity potential in this material is solved exactly by diagonalization and the conclusion obtained is in agreement with the general view point of other works. The values of the energy of ground state obtained by the exact solution and coherent state variation method is agree quite well.The qualitative characters of state vector given by both methods are consistent. The results shows that the coherent state variation method is valid and precise enough for this problem. Exact solution method will provide a basis to check the validity of any other approximate methods.