The present invention provides a novel inhibitor of FLT3 kinase, comprising a compound of formula (I) or a pharmaceutically acceptable salt, solvate, isomer, ester, acid, metabolite or prodrug thereof. The present invention also provides a pharmaceutical composition comprising the compound of formula (I), as well as the use and method for preventing or treating FLT3-related conditions, especially conditions related to mutant FLT3 kinase (particularly, FLT3/ITD mutant kinase).