LaTaON2 is a promising oxynitride photocatalyst for solar hydrogen production because of its intense visible light absorption. However, the presence of defect states and grain boundaries in conventional LaTaON2 restricts the improvement of photocatalytic performance. Here, an innovational platy LaTaON2 was prepared by a one-pot CsCl-assisted nitridation route, in which the in-situ formation of lattice-matchable CsLaTa2O7 precursor at early stage of nitridation facilitated the growth of shape-controlled platy LaTaON2 with high crystallinity. This platy structure promoted the diffusion of photogenerated charges carriers and allowed them to efficiently separate and transfer. When loading with a Pt cocatalyst, the platy LaTaON2 demonstrated a more than 6-fold improvement in photocatalytic activity for H2 evolution, compared with that fabricated with other fluxes or conventional LaTaON2. The visible-light-driven Z-scheme overall water splitting system was also successfully constructed. This finding confirms the effectivity of morphology and crystallinity control of oxynitride photocatalysts for promoting solar-to-hydrogen energy conversion.