To elucidate the microscopic mechanisms of pore structure evolution and external fluid interactions in siliceous shale reservoirs, systematic sensitivity experiments were conducted on siliceous shale reservoirs in the eastern margin of the Ordos Basin, focusing on external fluids such as variable-viscosity slick water and flowback fluid. Combined with the nuclear magnetic resonance (NMR) T₂ spectrum, the mechanism of action of variable viscosity slick water and flowback fluid on the pore structure of siliceous shale was studied. The evolution mechanism of pore throat size distribution was analyzed, and the difference in action mechanism between variable viscosity slick water and flowback fluid was clarified. The results show that: The variable viscosity of slickwater has a harmful effect on the total porosity of siliceous shale. The total porosity of siliceous shale can be reduced by about 6.8%. Among them, the decrease of macropores is the largest, followed by mesopores. The proportion of macropores and mesopores can be reduced by 46.2% and 7.4%, respectively, while the proportion of micropores shows an increasing trend. The total porosity of siliceous shale changes little after the action of flowback fluid. The change rate of total porosity after the action of flowback fluid is 0.04% and -0.02%. Among them, the proportion of medium pores increases, and the degree of change in micro-nano and small pores is small. The slickwater has weak damage to the total porosity of siliceous shale, micro-nano pores are not sensitive, small pores are not a sensitive-weak improvement, and large pores are moderately weak damage; the flowback fluid is not sensitive to the total porosity of siliceous shale and is not sensitive to micro-nano pores, small pores, mesopores, and macropores. The research results can provide a reference for fracturing process optimization and efficient development of shale oil.