Compound as selective jak inhibitor, and salt and therapeutic use thereof

Disclosed are a compound as a selective JAK inhibitor, and an isomer, a solvate or a pharmaceutically acceptable salt thereof, wherein: the definitions of（A）, R, R 1 and n are described in detail in the specification. In addition, also disclosed are a medicine comprising the compound and salts thereof as an active ingredient, and the use thereof in the preparation of a medicine for treating JAK-related target diseases, such as immune system diseases, rheumatoid arthritis and tumours.