The invention belongs to a method for preparing a high-purity MnBi alloy. The method is simple in process and comprises following steps: firstly, Bi powder and Mn powder are selected to serve as raw materials; secondly, the Bi powder and the Mn powder selected in the first step are evenly mixed according to a certain proportion and then are packaged in a molybdenum cup, and a molybdenum cup sample is subject to low-pressure pretreatment through a powder forming press or subject to ultrahigh-pressure pretreatment through a domestic six-side top liquid pressing machine; and thirdly, a molybdenum cup cavity obtained after being subject to prepressing treatment in the second step is placed in a crucible, and preparing of the MnBi alloy is completed through a low-temperature sintering and quenching program. The molybdenum cup is adopted to serve as the closed type cavity to be subject to low-temperature sintering, high-temperature volatilization of Bi3+ is effectively restrained, generation of Bi and Mn oxides is prevented, and purity of the MnBi alloy is improved; and the molybdenum cup has the high melting point, the higher oxidization temperature and high heat conductivity, and in the sintering process and the quenching process, the alloy is an excellent shielding material and an excellent heat conduction material.