when i check grids£¨double vertices and collapsed faces£©, there is always an error "collapsed faces found in model",i use command cflip to fix the problems but always failed, how can i deal with this trouble.

Are you sure that you're using CFLIP with the proper option? BTW, which option do you use?

You please check how you merged vertices after importing grids. If You have given more tolerance between the merged vertices then collapsed faces may come. Also check whether you used the option cset only while assembling grids. Then collapsed face problem may not arise.