Disclosed in the present invention is the compound as shown in formula I, or a stereoisomer thereof, or a pharmaceutically acceptable salt thereof, or a crystal form thereof, or a solvate thereof, or an isotope thereof. The compound in the present invention exhibits a good activity as a histone deacetylase inhibitor, has a significant inhibitory effect on cancer cells, and provides a new drug selection for clinically treating diseases related to abnormal histone deacetylase activity.
