The present paper proposes a method of linear stability analysis for subsonic disk MHD generators taking into account their loading condition and gasdynamical boundary condition. The flow in the MHD channel is described by quasi-one-dimensional equations, whose variational equations describe the behavior of perturbations of the flow. The time growth rate of the perturbations is determined by the characteristic equation which is derived from the gasdynamical boundary condition and the loading condition. Whether the perturbations grow or decay is judged diagrammatically in a way similar to the Nyquist method. The stability is analyzed of a coal-fired inflow subsonic disk MHD generator of commercial scale. The linear stability analysis and time-dependent calculations show that both the inlet condition and the loading condition much affect the stability. The generator often becomes un stable when the inlet swirl ratio is kept constant, whereas it is stable under the constant current loading condition or under the ohmic loading condition when the inlet azimuthal velocity is held constant.