I want to research the planar of free-form surface
But I don’t know where to start.
can you guide me.
If you mean relax a solution (i.e. division of a Surface/BrepFace and/or a quad Mesh) in order to achieve some quad planarity (up to some tolerance value) then study Kangaroo2. Lot’s of similar examples in this and the old (dead) Forum.
That said a quad planarity check is a check for the min diagonal distance.