The invention relates to a voltage building-up method in a guide vane pre-opening mode under a water pump working condition. When the working condition of a unit is changed into the water pump working condition from a pumped phase modulation working condition, the hydraulic vibration abnormal value of the unit with a small-opening guide vane is avoided in a guide vane pre-opening mode, so that the unit can be operated under the water pump working condition, and the performance requirement of the unit is met. The method has the advantages of convenience of use and high reliability, ensures that other units have sufficient water for debugging, has a positive effect on a pumped storage power station, and improves the project progress, safety stability and economic benefit of the power station.