The present invention relates to a super thick wall tadpole-like boron nitride nanometer powder preparation method, which comprises the following steps: (1) weighing raw materials such as sodium silicate, magnesium nitrate, and boron powder; (2) respectively preparing the sodium silicate and the magnesium nitrate into solutions with a certain concentration, adding the boron powder to the sodium silicate solution, adding ethanol in a dropwise manner, and carrying out ultrasonic stirring; (3) slowly adding the magnesium nitrate solution to the sodium silicate-boron powder mixing solution in a dropwise manner, and concurrently carrying out ultrasonic stirring, such that a grayish white colloid slowly appears; and carrying out standing suction filtration on the mixed solution after completing the addition, washing to achieve a neutral state, and placing the filter cake in a muffle furnace to carry out low temperature annealing to obtain a Si-Mg-O porous solid material; and (4) carrying out a reaction of the Si-Mg-O porous material and ammonia gas, and carrying out purification and drying to obtain the super thick wall tadpole-like boron nitride nanometer powder. According to the present preparation, the precursor preparation process is simple, energy consumption is low, and the precursor is porous, and can be directly used for mass preparation of the tadpole-like boron nitride nanometer powder without purification.