Based on ceramic laser gelling process, the purpose of this research is to obtain a proper slurry, which can be employed to cast layer with a thickness smaller than 100 μm to improve the staircase effect occurred in layer manufacturing, by optimization of the water content to fabricate ceramic part with better surface. The slurry was composed of alumina particles with average size of 0.5 μm as the structure element, silica sol as an inorganic binder and ammonium polymethacrylate as a dispersant. The slurry was dispersed uniformly by a ball mill. The results revealed the variation of water content will significantly influence the viscosity of the slurry and the formability of the ceramic green part. High water content led to poor binding of the particles and induced the delamination of the consolidated layers. On the contrary, low water content led to high shear force between scraper and slurry surface; such force damaged the previous layer during the casting process. As a result, the water content of 34 wt% was the most favorable to casting thin layers. The capability of casting 40 μm layer, which is smaller than that obtained by previous studies, was achieved. Because of the thinner layer, the staircase effect was improved.