A method for preparing lanthanum cuprate (La2CuO4) powder by means of the microwave-hydrothermal method includes the following steps: lanthanum nitrate is added into distilled water, so that solution A is obtained; copper nitrate is added into the solution A, so that solution B is obtained; citric acid is added into the solution B, so that solution C is obtained; the pH of the solution C is regulated to 1.5 to 7, so that solution D is obtained; the solution D is added into a microwave-hydrothermal reactor, the reactor is sealed and then put into a microwave-hydrothermal instrument to carry out hydrothermal reaction, and after reaction is finished, the reactor is naturally cooled to the room temperature; sample is collected by centrifugal separation and then repetitively washed by distilled water and acetone, and after drying, the La2CuO4 powder is obtained. Since the method adopts the microwave-hydrothermal method to prepare the La2CuO4 powder, the electric field and the magnetic field can be changed in the space, so that molecular motion can be intensified, as a result, heat energy can be obtained, and thereby the synthesis of the La2CuO4 powder becomes highly efficient, simple and highly repetitive. Moreover, the heating speed is high, heating is uniform and does not have the temperature gradient and the hysteresis effect, consequently, the defect of ununiform heating of a hydrothermal container is overcome, the reaction time is shortened, energy consumption is reduced, and the working efficiency is increased.