after simulation
I want the skirt keep the shape rather than dropping down like this
Maybe it can be solve by Cloth Stiffness Scaling option but I really don'n know how to use it?
Try increasing Bending and Structural. To have an order of magnitude, you can check the Presets :
Stiffness Scaling allows to control the stiffness of specific parts of the mesh. Create two vertex group (one for Struct, one for Bending), assign all your vertices to both groups, them weight-paint them.