The present invention relates to a method for preparing nanometer LaAlO3 powder by using a combustion method. The method comprises: controlling a raw material ratio, a pH value of a solution, and setting a calcination temperature and a constant temperature calcination time after self-propagating combustion in a combination manner to prepare fluffy powder-like nanometer LaAlO3 powder; mixing La(NO3)3.6H2O, Al(NO3)3.9H2O and C6H14O6 in a ceramic crucible according to a molar ratio of 1:1:(2-4), and adding an appropriate amount of distilled water; heating the mixed solution with a magnetic stirring apparatus having a heating function to a temperature of 70 DEG C, and uniformly stirring; slowly adding ammonia water in a dropwise manner to adjust the pH value of the mixed solution to 2-4; placing the solution in a muffle furnace to heat until self-propagating combustion occurs; and carrying out calcination for 3-5 hours at a temperature of 750-950 DEG C. According to the present invention, a reaction time is short, and dramatic impact of the combustion gas stream is generated, such that particle refinement is easily achieved, and the degree of agglomeration between the particles can be reduced so as to obtain the fluffy powder-like nanometer LaAlO3 powder with an average particle size of 20-40 nm.