Nanosilica (NS) has been introduced to enhance the early strength and mitigate the porous structure issues associated with high-volume ground blast furnace slag (GGBS) cementitious materials (HVS). The impact of NS on the sulfate attack resistance in HVS cementitious materials is a noteworthy concern, especially concerning the degradation evolution process under sulfate attack. This study comprehensively analyzed the total volume changes and in-situ pore structure evolution for NS modified HVS cementitious materials under sulfate attack. The findings revealed that NS could effectively cause an increase in pores smaller than 100 nm and decrease in pores larger than 100 nm in HVS cementitious materials. Specifically, NS contributed to a reduction in volume expansion, weight loss, and surface damage of 80 wt% GGBS cement paste during sulfate attack. NS demonstrated its efficacy in improving the ability of 80 wt% GGBS cementitious materials to withstand expansion pressure caused by sulfate attack products, though NS did not significantly reduce the quantity of sulfate attack products. Conversely, NS led to an increase in volume expansion, weight loss and early damage of 60 wt% GGBS cement paste. The damage inflicted on cement paste under sulfate attack primarily manifested as the destruction of pores in nanometer. The process involved a cyclic sequence of pore filling, damage, refilling, and redamage during the sulfate attack process.