Hi, Kirk. 
Thank you for your suggestion and encouragement.
I used dcm2nii to generate bvec file before. But it doesn't work. And that's why I begin to doubt what's exactly wrong about my data. Then I used the data that I changed using Matlab based on bvec files, and the results are the same. So I think I have only one problem left which I stated a minute ago. But I will try your suggestion too and see how it goes. It's just that I don't have enough background knowledge of these softwares. It takes really long for me to understand what exactly that means.
Best regards,
Xinman