The invention discloses a beta-phosphorus building plaster preparing method. The method comprises the steps of screening ardealite firstly, and then mixing the screened ardealite with water and citric acid in certain proportion to obtain a mixed material; conducting stirring and standing in sequence repeatedly till floating objects on the surface of the mixed material are completely removed; adding Ca(OH)2, conducting stirring and standing till the pH value of the mixed material is 7-10, obtaining a discharged material, conducting heating and drying, and obtaining beta-phosphorus building plaster through secondary screening after the discharged material is naturally cooled to normal temperature. By conducting secondary screening and adding a habit modifier, impurities such as P, F and organic matter can be effectively removed, the crystal shape of the ardealite is converted smoothly, and then the high-strength beta-phosphorus building plaster is obtained. By the adoption of the method, the usage and number of varieties of chemical reagents are reduced, the problems of traditional beta-phosphorus building plaster production methods that a large number of chemical reagents are added, the production process is complicated, and secondary pollution is serious are solved, and production cost is saved.