The present invention relates to a novel compound as a JAK inhibitor, a composition, and an application thereof. Specifically, the present invention provides a compound having high AK inhibitory activity (as represented by formula (I)) or its isomer, solvate, hydrate, pharmaceutically-acceptable salt, and prodrug, and a pharmaceutical composition containing the compound. Also disclosed is an application of the present compound or pharmaceutical composition in preparation of a drug, and the drug is used for treating autoimmune diseases or cancers.