KNbO3 (KN) piezoelectric polycrystals were prepared by a two-step solvothermal reaction process with the managed organic solvents as reaction mediums at a low temperature for a short time. In the solvothermal reaction system, the formation mechanism of polycrystalline KN is mainly the dissolution-deposition mechanism. The influences of alkalinity, viscosity, and the polarity for reaction mediums on the formation of the niobates were investigated. The chemical reaction mechanisms of niobate products and formation mechanism of niobate crystals from the precursor were clarified. The regulating and controlling mechanism of the phase compositions, the morphologies, and the lattice constants for the niobates obtained in varied reaction mediums were revealed. The obtained KN piezoelectric polycrystals are constructed from oriented KN nanocrystals. Piezoelectric hysteresis loops of cuboid KN polycrystals were detected for the first time. A prepared cuboid KN polycrystal shows an average d(33)* value of 32 pm/V. The study provides a strategy for the development of oriented KN piezoelectric materials to apply the orientation engineering.