Dear Ruiliang,
The mesh-mapping functionality of FAST permits each module to have its own statial discretization and spatial extent. This functionality allows one to transfer motions and loads between ElastoDyn, which assumes an initially straight blade, and AeroDyn v15, which permits the modeling of precurved and preswept blades. See the following papers for more information on the mesh-mapping functionality of FAST: nrel.gov/docs/fy16osti/63203.pdf.
There are several topics on this forum that have discussed how to set up BeamDyn to model precurved or preswept blades e.g. see: pre-bended & sweep blade simulation with BeamDyn &aerodyn 15 - #4 by Jason.Jonkman. Have you already followed this advice? Also, which version of FAST are you using? There have a been a number of improvements in BeamDyn and AeroDyn in the upgrade from FAST v8.16 and OpenFAST v1.0.0 and newer.
Best regards,