例如，我要生成一个16位的字符串。要求混合数字，大写字母，小写字母。完全随机。存入变量a就行，不用显示到标签里。

以下简单的程序可实现（每次按command1生成一组）

Private Sub Command1_Click()
Randomize '初始化随机函数发生器，没有它，每次生成随机数相同
For i = 1 To 16
x = Int(Rnd() * 3)
If x = 0 Then
a = a + Chr(Int(Rnd * 10) + 48) '数字 ASCII为48到58
Else
  If x = 1 Then
     a = a + Chr(Int(Rnd * 26) + 65) '大写
  Else
    a = a + Chr(Int(Rnd * 26) + 97) '小写
  End If
End If
 
Next
Print a
 
End Sub

测试结果

Randomize 

 For i = 1 To 16 '字符串的数量 

 Select Case Rnd * Rnd * 35

 Case Is < 1 

 b = "a"

 a = a & b

 Case Is < 2 

 b = "b" 

 a = a & b

 Case Is < 3

 b = "c"

 a = a & b 

 Case Is < 4 

 b = "d" 

 a = a & b

 Case Is < 5 

 b = "e" 

 a = a & b 

 Case Is < 6 

 b = "f" 

 a = a & b 

 Case Is < 7 

 b = "g" 

 a = a & b 

 Case Is < 8 

 b = "h" 

 a = a & b

 Case Is < 9 

 b = "i" 

 a = a & b 

 Case Is < 10

 b = "j" 

 a = a & b

 Case Is < 11

 b = "k" 

 a = a & b 

 Case Is < 12

 b = "l" 

 a = a & b 

 Case Is < 13

 b = "m" 

Case Is < 14 

 b = "n"

 a = a & b

 Case Is < 15

 b = "o" 

 a = a & b 

 Case Is < 16

 b = "p" 

 a = a & b 

 Case Is < 17 

 b = "q" 

 a = a & b

 Case Is < 18 

 b = "r" 

 a = a & b 

 Case Is < 19

 b = "s" 

 a = a & b 

 Case Is < 20

 b = "t"

 a = a & b

 Case Is < 21 

 b = "u" 

 a = a & b

 Case Is < 22

 b = "v"

 a = a & b 

 Case Is < 23 

 b = "w" 

 a = a & b

 Case Is < 24

 b = "x" 

 a = a & b 

 Case Is < 25

 b = "y" 

 a = a & b

 Case Is < 26

 b = "z" 

 a = a & b

 Case Else

 b = "。"

 a = a & b

 End Select 

 Next i

'vb6写过来的，自己写的，虽然笨……vb6粘贴过来还没空行，又要一个一个空行……

'以上代码可以生成小写的26字母和句号，随机的，放在变量a中。亲测可用。

'如果需要其他的，那就在单分支语句里加，我写不下去了。

'望采纳！