Objective To explore the mediating effect of sleep quality on the relationship between cerebral small vessel disease （CSVD） and cognitive function among the older adults in Nantong. Methods A total of 412 patients aged 60 years or above with CSVD admitted to the Department of Nantong First People's Hospital in Nantong from March 2024 to March 2025 were enrolled in this retrospective study. Based on the cognitive function assessed by the Montreal Cognitive Assessment （MoCA）， patients were divided into the cognitive impairment （CI） group and the non-CI group for comparison. Sleep quality was evaluated using the Pittsburgh Sleep Quality Index （PSQI）. Multiple linear regression analysis was conducted to determine the relationships among key variables. The mediating effect of sleep quality between CSVD and cognitive function was analyzed using Model 4 of the PROCESS macro version 4.1， and the mediating effect was validated through the Bootstrap method. Results There were 192 patients （46.6%） with CI， and 220 patients （53.4%） without CI. Patients in the CI group presented a greater burden of CSVD， higher proportions of lacunes， moderate-to-severe white matter hyperintensities （WMH）， cerebral microbleed （CMB）， and basal ganglia-enlarged perivascular spaces （BG-EPVS） exceeding 10， as well as elevated PSQI scores compared to the non-CI group. All these differences were statistically significant （all P < 0.05 or P < 0.001）. After adjusting for covariates such as age， sex， years of education， Hamilton Anxiety Scale （HAMA） scores， Hamilton Depression Rating Scale-17 （HAMD-17） scores， and vascular risk factors， the CSVD burden， the number of lacunes， the number of CMB， the number of BG-EPVS， and the severity of WMH were all significantly negatively correlated with Montreal Cognitive Assessment （MoCA） scores （all P < 0.001）， and significantly positively correlated with PSQI scores （all P < 0.05 or P < 0.001）. Additionally， PSQI scores were significantly negatively correlated with MoCA scores （P < 0.001）. Mediated effect analysis indicated that sleep quality plays a partial mediating role in the relationship between CSVD and cognitive function. The mediating effect ratios in the relationships between CSVD burden， lacunes， WMH， CMB， BG-EPVS and cognitive function are 15.2%， 15.8%， 21.7%， 16.7% and 23.0% respectively. In the Bootstrap test （5 000 times）， neither the upper nor the lower limit of the 95% confidence interval contains 0， suggesting that the mediating effect is significant. Conclusions Sleep quality plays a partial mediating role in the process by which CSVD leads to CI among the elderly in Nantong. Enhancing sleep quality may be beneficial in reducing the risk of developing CSVD-CI.