The invention relates to a four-quadrant reactive power control method for an excitation system of a pumped storage unit, which comprises the following steps of: a step of carrying out over-excitation limitation; a step of carrying out underexcitation limitation; and a step of controlling to superpose a given voltage Uref and a terminal voltage Ut by an over-excitation and underexcitation limitation output and a main ring of an excitation regulator, then outputting an excitation voltage Uf after calculating and limiting the amplitude by PID (Proportion Integration Differentiation) and controlling the size of an excitation current, so that the over-excitation and underexcitation limitation on a motor is realized. The four-quadrant reactive power control method has the beneficial effect that the pumped storage unit can limit a reactive power of the motor under all the working conditions of generation lagging phase operation, generation leading phase operation, electric lagging phase operation and electric leading phase operation.