The invention discloses a method for preparing a compact nanocrystalline WC-Co hard alloy block material with a small grain size, which belongs to the technical field of novel materials and novel powder metallurgy. The method comprises the steps of: performing mixed ball milling on a grain growth inhibitor, namely VC powder particles subjected to ball milling refinement treatment and nanometer WC-Co composite powder to obtain nanometer WC-Co-VC mixed powder of which the components are distributed uniformly and which has high dispersibility; and quickly sintering the nanometer WC-Co-VC mixed powder by spark plasma sintering technology for compacting, and finally obtaining the compact nanocrystalline WC-Co hard alloy block material with the small grain size by applying a novel process formed by the links such as ultra-fast heating at the initial stage, pressurization after gas discharge, sintering temperature reduction, short-time heat preservation by stages and the like. The nanocrystalline hard alloy block material prepared by the method has the advantages of pure phase, compactness, small WC grain size and the like. The method has the remarkable advantages of simple steps, short flow, energy conservation and environmental-friendliness and the like.