This looks nasty to me Xue,
It is in the location of the Gluteus Maximus insertion.
There is bony involvement.
It could be infective like osteomyelitis or an abscess forming in the G-Max with bony involvement.
It could also be a malignant process like a metastasis or q primary bone tumour.
It needs cross sectional imaging to assess the femur and overlying soft tissues,
Either way I don’t like the look of it!
Let us all know what the cross sectional imaging shows.
Steve