With the detailed model of the wind farm taken into account, it is difficult to achieve reactive power margin sharing by allocating reactive power according to the reactive capacity proportions of the Doubly-Fed Induction Generators (DFIG). According to the reactive power margin of the DFIG and the tolerable voltage deviation at the Point of Common Coupling (PCC), an improved reactive power-voltage droop control incorporating the variable droop coefficient is proposed. Combining the reactive power margin of the DFIG and electrical distance between DFIG and PCC, a novel indicator of the reactive power imbalance is proposed. To deal the control problems of large-scale wind farms, a bi-level reactive power optimization model is established. For the grid level, the reactive power demands of the wind farms that minimize the grid loss, voltage deviation, and copper loss of the wind farms are determined. For the wind farm level, the reactive power sharing among the DFIGs is determined with the control object to minimize the circuit loss of the wind farm, the copper loss of each DFIG, and the reactive power imbalance. The LBFGS-TR optimization algorithm is adopted to solve the reactive power sharing scheme. Simulation results show that the proposed algorithm helps to realize the reactive power margin sharing of the wind farms with the fully utilized reactive power regulation capability of the DFIG. This work is supported by National Natural Science Foundation of China (No. 51877061).