import arcpy, os
#workspace to search for MXDs
Workspace = r"C:\Geo"
arcpy.env.workspace = Workspace
#list map documents in folder
mxdList = arcpy.ListFiles("*.mxd")
#set relative path setting for each MXD in list.
for file in mxdList:
#set map document to change
filePath = os.path.join(Workspace, file)
mxd = arcpy.mapping.MapDocument(filePath)
#set relative paths property
mxd.relativePaths = True
#save map doucment change
mxd.save()