As an important function of the high voltage direct current (HVDC) transmission system, the reactive power control function is used in Yunnan-Guangdong ± 800 kV ultra-high voltage direct current (UHVDC) transmission system just like in the traditional HVDC system. In addition, the reactive power backup control function is used in the UHVDC system as a supplement. The control strategy and realizing method of reactive power backup control function are analyzed. It is pointed out that when the control-bus communication of both the pole control and the DC station control becomes faulty, the reactive power backup control function is found defective. An improvement proposal is made and applied on the spot in the Yunnan-Guangdong ± 800 kV UHVDC transmission system. © 2011 State Grid Electric Power Research Institute Press.