Nano-materials have proven to be successful in improving hydrocarbon recovery in the unconventional oil and gas reservoir, but the mechanism of their adsorption and retention in the pores is not yet clear. Taking the tight sandstone of Qingshankou Formation of Upper Cretaceous in Daqing Oilfield as the research object, the adsorption and retention of nano-slickwater fracturing fluid in pores and its effects on microscopic pore structure were studied by means of low-temperature nitrogen adsorption, wetting angle measurement, scanning electron microscopy, nuclear magnetic resonance and centrifugal experiment. The results show that the nano-particles are retained in the pores and the wetting angle of the rock decreases by 30.28 % ~ 58.17 % after the nano-slickwater fracturing fluid treatment. The pore structure transits from flat hole to ink bottle hole, and the specific surface area and adsorption amount increase significantly. The pore structure transitions from the plate-like to the ink-bottle-like, and the specific surface area and adsorption amount increase significantly. The proportion of micropores decreases by 20%~25%, the proportion of transitional-pores increases by 21%~26%, and the total pore volume increases. The fractal dimension becomes smaller and closer to 2, and the pore structure becomes simpler. The adsorption and retention of nanoparticles in the pores of the reservoir lead to changes in the microscopic pore structure. The experimental results and understanding are of great significance for the application of nano-slickwater fracturing fluid in tight sandstone reservoirs.