Ag nanoplates, as two-dimensional plasmonic nanostructures, have attracted intensive attention due to their strong shape-dependent optical properties and related applications. Here parallel face-exposed Ag nanoplates vertically grown on micro-hemisphere surfaces have been achieved by firstly electrodepositing the micro-hemispheres assembled by Ag nanoplates, whose planar surfaces are stuck together, on indium tin oxide substrates, and then Ostwald ripening the as-electrodeposited micro-hemispheres in water. The sizes of the nanoplates and the gaps between the neighboring nanoplates have been tailored by tuning the Ostwald-ripening duration, so that the SERS activity of the micro-hemispheres has been remarkably improved. The improved SERS activity can be well explained by our systematic finite-element simulation. Therefore, Ostwald ripening offers a route to the synthesis of Ag nanoplates, and the optimization of plasmon coupling and SERS activity of nanostructure-assembled systems.