The invention belongs to the technical field of medicine biologics, relates to functions and mechanisms of PCSK9 (Proprotein Convertase Subtilisin/Kexin 9) in treating T-cell medicated inflammatory-immune diseases, application of a PCSK9 inhibitor in preparing a medicine for treating T-cell medicated inflammatory-immune diseases, and in particular relates to application of a PCSK9 small-molecule interference RNA or PCSK9 small-molecule inhibitor in preparing medicines for preparation systems or external skin application in treating psoriasis, atopic dermatitis or urticaria. Psoriasis is adopted as a testing bed for inflammatory-immune disease research, results show that the PCSK9 small-molecule inhibitor or small-molecule interference RNA has treatment effects which are remarkably prior tothose of PCSK9 monoclonal antibodies in treating inflammation such as psoriasis. The PCSK9 small-molecule interference RNA or PCSK9 small-molecule inhibitor can be further developed into novel medicines for treating inflammatory-immune diseases such as psoriasis, and is small in side effect, low in cost and remarkable in treatment effect.