The invention relates to an outsourcing large-volume concrete pressure-preserving pouring method below a volute belt line of a hydropower project. The method conducts partition block compartment pouring on the concrete below the volute belt line, carries out outsourcing large-volume concrete pouring under the volute belt line in a divided first area, carries out the concrete pouring on an internalcorner part in a divided second area, and carries out the backfill grouting on an internal corner part in a divided third area. The method also adopts temperature control measures to improve the pouring quality of the concrete, adopts maintenance measures to ensure the pouring quality of the poured concrete, and conducts monitoring and feedback analysis on the whole process of a volute, a seat ring, the pouring concrete and the grouting. The method is suitable for the construction of the large-volume concrete pouring in a volute layer of an underground powerhouse of the hydropower project, improves the pouring quality of the concrete, and ensures the stable operation of the volute.