The invention discloses a microwave preparation method of alpha-calcium sulfate hemihydrate. The method includes the steps of: adding calcium sulphate dehydrate into an aqueous solution containing calcium chloride and sodium chloride, carrying out microwave heating reaction under the action of ultrasound, and conducting filtering, washing and drying to obtain alpha-calcium sulfate hemihydrates. The method provided by the invention is characterized in that it utilizes the synergistic effect of microwave and ultrasonic wave to accelerate the reaction rate and reduce the reaction time, and the prepared alpha-calcium sulfate hemihydrates crystal is a fibrous structure, and has uniform shape and size and high crystallinity.