The invention provides a method for preparing monolayer or oligomeric boron nitride nanosheets with concentrated sulfuric acid as an intercalation agent. The method comprises the following steps: uniformly mixing and stirring commercially available block boron nitride raw materials with concentrated sulfuric acid at room temperature, enabling the concentrated sulfuric acid to enter into boron nitride layers to obtain a uniformly mixed solution; rapidly pouring the uniformly mixed solution into quantitative ultrapure water, and conducting ultrasonic dispersion to obtain a boron nitride dispersion solution; carrying out suction filtration on the boron nitride dispersion solution and washing until the pH value of suction filtration liquid is neutral, and then dispersing residues evenly in the ultrapure water to get a uniform dispersion solution; standing the uniform dispersion solution, removing large unstripped boron nitride, and taking supernatant liquid to obtain a boron nitride nanosheet dispersion solution; separating boron nitride nanosheets from the boron nitride nanosheet dispersion solution, and drying to obtain the product boron nitride nanosheets. The method for preparing the monolayer or oligomeric boron nitride nanosheets with the concentrated sulfuric acid as the intercalation agent has the characteristics of being simple in reaction equipment, short in reaction time, low in cost, high in yield, environmentally friendly, easy in scale production, good in product quality and the like.