Hi Yeping Sun,
A linear polymer is likely to apparent when you look at a large number of symmetry mates. Looking at lots of symmetry mates in cartoon view is quite intensive and can be slow unless you have a powerful graphics card - if you show the first molecule as 'ribbon' and then generated symmetry mates it can lead to a better viewing experience.
If you tell me the pdb code I can have a look.
Institute of Microbiology, Chinese Academy of Sciences