VB编程,请用循环语句,mid等演示给我看

不是很明白你的意思,是不是要对纯字母的字符串进行转换,比如AgyZdhFSH转换为FldEimKXM?
asc(c)与asc("c")肯定是不同的,前面的c是变量,可以是26个字母中任一个,而后面的"c"就是小写的c.
写了个程序,测试通过,不知道是不是你要的.窗体放一个CommandButton和三个TextBox,第一个文本框输入要转换的字符串,第二个输入k值,第三个输出转换后的字符串.
Private Sub Command1_Click()
Dim t1 As String, t2 As String, c As String
Dim i As Integer, k As Integer, n As Integer
t1 = Text1.Text
k = Text2.Text
For i = 1 To Len(t1)
c = Mid(t1, i, 1)
n = Asc(c) + k
If (n > 90 And n < 97) Or n > 122 Then n = n - 26
c = Chr(n)
t2 = t2 & c
Next
Text3.Text = t2
End Su