Wind-thermal-bundled power transmission by UHVDC system is regarded as one of feasible ways on development of global clean energy in the future and its characteristics of stability should be paid attention to. Different wind-thermal installed capacity ratio is proposed with the wind from the change of power angle and in consideration of the probability for wind power output. The effect of two kinds of proportion on the stability of the sending end islanded system is comparatively analyzed by the simulation. Regulating ability of sending end power and equivalent mechanical rotational inertia would be lower when wind-thermal installed capacity ratio decreased from 1:2 to 1:1.5. Under DC power system disturbances and loss of parts of power, there is a slight increase in ascending amplitude of system frequency and power-angle fluctuation, risk of system destabilization will rise accordingly.