The present invention relates to a ubiquitin-specific peptidase 24 inhibitor, a medicinal composition and a method of delaying or reversing multidrug resistance in cancers using the same. The USP24 inhibitor, which includes a shUSP24 RNA and/or a carbonyl substituted phenyl compound, can serve as a chemosensitizing agent for inhibiting the drug pump out, cancer stemness and genomic instability of cancer cells, thereby being applied to a medicinal composition and a method for delaying or reversing multidrug resistance in cancers.