The invention relates to the field of drug research and development and discloses a gamma-secretase inhibitor and a use thereof. The invention creatively discloses a compound shown in the formula I or its pharmaceutical acceptable salt through a test. The compound can effectively inhibit gamma-secretase activity on a concentration mmol/L level, obviously reduces a cell level A beta amount, does not influence a NOTCH shearing process and has good specificity. The compound shown in the formula I or its pharmaceutical acceptable salt can be used for preparation of the gamma-secretase inhibitor or a drug for treating Alzheimer's disease, are safe and reliable and have a good clinical application prospect.