The invention discloses a method for synthesizing mercurous iodide. The method comprises the steps of: S1, cleaning and drying an ampoule for future use; S2, putting the measured mercury and mercury iodide into the ampoule prepared in the step S1, vacuumizing the ampoule and sealing the ampoule when the vacuum degree is reduced to the range from 10<-2> to 10<-3> Pa; S3, vertically fixing the ampoule in the step S2 in a reaction furnace, and increasing the temperature of the reaction furnace to the range from 300 to 370 DEG C at a temperature rise rate of 20-100 DEG C per hour; S4, after all the mercury iodide is melted, standing for 1-4 h; S5, reducing the temperature of the reaction furnace to any temperature under the syntectic reaction temperature of an iodine-mercury binary system and above the eutectic reaction temperature of the iodine-mercury binary system at a temperature reduction rate of 200-400 DEG C per hour, preserving heat for 2-10 h and then cooling to the room temperature; and S6, taking out the obtained product in the ampoule and removing mercury at the bottom and mercury iodide at the top. The method for synthesizing the mercurous iodide is capable of obtaining the mercurous iodide in which the ratio of mercury atoms to iodine atoms is 1: 1; and the crystallization temperature range is wide and the method is easy to control.