The invention relates to a preparation method of semi-hydrated gypsum, comprising the following technical steps of: delivering dihydrate gypsum used as raw material into a rotary type pan with the rotary speed of 0.5 r/minute-0.7 r/minute; calcining the dihydrate gypsum at the high temperature of 170 DEG C-190 DEG C, wherein the calcining time ranges from 8 minutes to 12 minutes; reducing the temperature of the pan to 140 DEG C-150 DEG C; further drying and dewatering the dihydrate gypsum at low temperature for 58 minutes to 65 minutes; and taking out of the pan after the drying is completed so as to obtain the semi-hydrated gypsum with the purity more than 95 percent. When in use, the semi-hydrated gypsum is convenient to use due to longer condensing time and has higher strength.