Abstract: SIL1, an ER-resident protein, has been reported to play a protective role against Alzheimer’s disease (AD). However, the effect of SIL1 on APP processing remains unclear. In this study, we explored the role of SIL1 in APP processing both in vitro and in vivo. In the in vitro experiment, SIL1 was overexpressed or knocked down in cells stably expressing the human Swedish mutant APP695. In the in vivo experiment, AAV-SIL1-EGFP or AAV-EGFP was microinjected into the APP23/PS45 mice and their wild-type littermates. Western blotting (WB), immunohistochemistry, RNA-seq, and behavioral experiments were performed to evaluate the relevant parameters. We found that SIL1 expression decreased in APP23/PS45 mice. Overexpression of SIL1 significantly decreased the protein levels of APP, PS1, and CTFs both in vivo and in vitro. Conversely, the knockdown of SIL1 increased the protein levels of APP, BACE1, PS1, and CTFs, as well as the mRNA expression level of APP in 2EB2 cells. Further, the overexpression of SIL1 reduced the number of senile plaques in APP23/PS45 mice. Importantly, the Y-maze and Morris Water maze tests showed that the overexpression of SIL1 improved cognitive impairment in APP23/PS45 mice. Our findings demonstrate that SIL1 improved the cognitive impairment of APP23/PS45 mice by inhibiting APP amyloidogenic processing, and suggest SIL1 as a potential therapeutic target for AD therapy by modulating APP processing.