string strI46="D:\Basen\工作\质量检查\Data\I46\I46E003021\DLG\卫片\I46E003021.mdb ";
string strI46=strI46.substring(strI46.Indexof("\I46\")+1);

------解决方案--------------------
C# code

 string s = @"D:\Basen\工作\质量检查\Data\I46\I46E003021\DLG\卫片\I46E003021.mdb ";
 s = s.Substring(s.IndexOf("I46"));