A heterocyclic compound is described, which is an inhibitor of FGFR (fibroblast growth factor receptor). Specifically, it is a compound represented by the following formula (I), including an isomer (enantiomer or diastereomer) which may be present, or a pharmaceutically acceptable salt thereof, prodrugs, deuterated derivatives, hydrates, solvates. The definition of each group in the formula (I) is as described in the specification. The compound of the present invention has FGFR inhibitory activity and can be used for preventing or treating a disease associated with FGFR activity or expression.