We point out in this paper that the heavy Fermion system is a kind of Kondo system whose localized f-electrons can change with temperature. The theory suggests the existence of spontaneous magnetic moment in particular temperature range. According to the different distance between the f-electron band and Fermi level, there are different types of spontaneous magnetic moment effect. We also point out that the anomaly in specific heat versus temperature-curve is caused by the change of number of f-electron in heavy Fermion system. The theoretical values of specific heat coincide well with six experimental data.