Water soluble core has been widely used in manufacturing complex metal components with hollow configurations or intal channels; however, the soluble core can absorb water easily from the air at room temperature. To improve the humidity resistance of the water soluble core and optimize the process parameters applied in manufacturing of the water soluble core, a precipitation method and a two-level-three-full factorial central composite design were used, respectively. The properties of the cores treated by the precipitation method were compared with that without any treatment. Through a systematical study by means of both an environmental scanning electron microscope (ESEM) and an energy dispersive X-ray (EDX) analyzer, the results indicate that the hygroscopicity can be reduced by 20% and the obtained optimal process conditions for three critical control factors affecting the hygroscopicity are 0.2 g.mL-1 calcium chloride concentration, 4% water concentration and 0 min ignition time. The porous surface coated by calcium chloride and the high humidity resistance products generated in the precipitation reaction between calcium chloride and potassium carbonate may contribute to the lower hygroscopicity.