A reactive power control method, a reactive power control device, and a reactive power control system are provided. The device includes: a communication interface, configured to receive a reactive power requirement command; an input interface, configured to acquire an electrical quantity parameter of a preset control point; a strategy calculation module, configured to calculate a target reactive power value meeting the reactive power requirement command based on the electrical quantity parameter, and allocate reactive power to be provided by a regulatable reactive device based on the target reactive power value; and an output interface, configured to send a command for providing the allocated reactive power to the regulatable reactive device.