The invention discloses a method of controlling a dihydrate gypsum for preparing alpha-hemihydrate in salt solution. A curve of electrical conductivity with the time is obtained by measuring electrical conductivity of serosity in alpha-hemihydrate preparing process with actual prepartion of same dihydrate gypsum. According to the curve to judge phase change in crystallization transformation process of the dihydrate gypsum and determine the relationship between the transformation extent of the dihydrate gypsum to the alpha-hemihydrate and the electrical conductivite; setting a electrical conductivity in production process to control transformation extent of the dihydrate gypsum to the alpha-hemihydrate. The invention provides a convenient and simple, easy and accurate indicating and controlling method for salt solution method of using the dihydrate gypsum to prepare the alpha-hemihydrate, which can test both on-line and off-line, and being suitable for technics control in industrial production process.