The invention provides an online dynamic balance control method and system for a spaceborne rotary camera. The method comprises steps that after keeping the present position of a balance head relative to the spaceborne rotary camera, adjusting a relative angle of each balance block of the balance head relative to the spaceborne rotary camera at a first angle, adjusting a relative angle of one balance block of the balance head relative to the spaceborne rotary camera at a second angle and adjusting a relative angle of another balance block of the balance head relative to the spaceborne rotary camera at a third angle, measurement is respectively carried out, and the vibration information corresponding to a movement state of online movement of the spaceborne rotary camera is measured; based on the vibration information, vibration amount and fixing amount corresponding to each piece of vibration information are calculated, lastly, on the condition that an initial angle of the balance block is not clear, adjusting amount of the balance blocks relative to the present angles is simply determined, and online movement of the spaceborne rotary camera is adjusted to reach balance requirement conditions.