The characteristics of clustering wind farms are strongly coupled between reactive power and voltage of wind farms and poor communication reliability. Based on these characteristics，this paper proposes a graded voltage control strategy. In the control strategy，doubly-fed induction generators（DFIGs）are main reactive power resources because of their high reliable reactive power generation ability and capacitors/static var compensators（SVCs）are auxiliary reactive power resources. This makes the strategy independent of high speed and high reliability communication between DFIGs and wind farm automatic voltage control（AVC）system. According to the grid operation condition，the PQ curve of DFIG is calculated previously. Based on the pre-setting PQ curve，the reactive power output of DFIG is adjusted automatically to eliminate voltage fluctuation caused by change of active power output. Only when the grid operation condition changes significantly，the PQ curve need to be updated，that means the PQ curve need not be calculated in real time. When the voltage control aim is not achieved or the updating of PQ curve does not finish，capacitors/SVCs are then used to control voltage，which is called the secondary voltage control. Simulations on a real clustering wind farms in northern China is carried out. The results show that the probability of wind generatorscascading disconnection from power grid is reduced effectively with the proposed graded voltage control strategy. Communication failures between DFIGs and wind farm AVC system have no influences on control effect.