The invention discloses a method for preparing calcium sulfate whisker from phosphogypsum. The method comprises the following steps: (1) crushing and grinding phosphogypsum serving as a raw material to obtain phosphogypsum powder; (2) mixing the obtained phosphogypsum powder and diluted acid to obtain a mixture, placing the mixture in a reactor with a stirring device, reacting by controlling a certain temperature to obtain an acidic mixed solution containing calcium, sulfate ions and insoluble matters; (3) filtering the obtained mixed solution while the solution is hot to obtain an acidic filtrate; and (4) cooling the obtained acidic filtrate and crystallizing, filtering, washing and drying to obtain the calcium sulfate whisker product. The calcium sulfate dihydrate whisker with large length-diameter ratio is directly prepared from the raw material by virtue of one-step acid leaching impurity removal method instead of pretreatment impurity removal and thus the advantages that the process flow is simple, the production cost is low, the energy consumption is small and the acidic wastewater and washing water can be recyclable are achieved; the obtained calcium sulfate whisker has the characteristics of high purity, large length-diameter ratio and the like and is conductive to popularization and application.