The effect of injection fuel energy on the plasma dynamics stability of a stationary fusion reactor has been investigated, using a mathematical model based on the energy density and particle density balance of four plasma constituents. For this purpose, numerical calculations of eigenvalues have been made after the linearization of a set of plasma equations. The result suggests that a neutral-beam injection gun of moderate energy favours, from the point of view of dynamics, the realization of a stable plasma rather than a pellet injection method of very low energy. Further calculations have been made to study the inner mechanism of thermal instability of a plasma. Finally, a feasible way of plasma stabilization by feedback control has been examined.