Abstract: Quantity of water-adding in carnallite decomposition process determines the conversion and yield of  potas-sium,and quantity of water-adding can be calculated by magnesium content in carnallite.Quantity of water-adding should be adjusted with the changing of carnallite grade.In general,the suitability of water-adding was estimated by analyzing ionic composition of solid phase and liquid phase in carnallite decomposition process,but the analysis detection time in this process was too long.Influences of different quantitues of water-adding on moisture content and Mg content in solid phase of carnallite decomposition process were studied,and carnallite decomposition status can be ensured by using the relationship of Mg content and moisture content in solid phase of decomposition process.Result showed that Mg content and moisture content in solid phase had nonlinear variation when water-adding was insufficient;too much water Mg content and moisture content in solid phase had linear change.This method for confirming the suitability of water-adding quantity by the relationship of Mg content and moisture content in solid phase of carnallite decomposition process is simple,fast,and effective.