问题解决了，原来是我数据库PIC字段的长度设太长了，导致出错。
正确做法：
Sub DeleteAFile(filespec)
Dim fso
Set fso = CreateObject("Scripting.FileSystemObject")
fso.DeleteFile(Server.MapPath(filespec))
End Sub
sql="select * from web_Certificate where ID="&id
call xf.creaters(rs,sql,1)
add=rs("pic")
call DeleteAFile(add)