The invention discloses a wind power plant constant voltage control method which comprises the following steps: collecting active power real-time value, reactive power real-time value and voltage real-time value; obtaining reactance, voltage set value and reactive power output rated value; carrying out calculation to obtain an active current component according to the active power real-time value and the voltage real-time value, and carrying out calculation to obtain a reactive current component according to the reactive power real-time value and the voltage real-time value; obtaining a voltage reference value according to the reactance, voltage set value, the active current component and the reactive current component; comparing the voltage reference value with the voltage real-time value to obtain voltage difference; carrying out calculation to obtain a reactive power output initial value according to the voltage difference; comparing the reactive power output initial value with the reactive power output rated value to obtain a reactive power compensation instruction; and finishing reactive power compensation according to the reactive power compensation instruction. According to the wind power plant constant voltage control system and method, characteristics of wind power plant control is combined so as improve the wind power dispatching automation control level, and voltage stabilization level of a power grid is improved.