读TXT文本文件，读到满足条件的语句就截取字符串，字符串的格式类似于“123.4567891234”（小数点前3位，小数点后最少1位，最多8位），
想对此数据累加，所用的语句为 a = a + VAL（字符串），这样对吗？怎么最后得出来的数字为整数，无小数？

您把整数和小数分开，再修改以下代码即可。
Private Sub Command1_Click()
  test
End Sub
Sub test()
sgg = BigNumAdd("1230981204973453434534510732", "64569845695683295334534534534586")
  MsgBox sgg
End Sub

'相加
Public Function BigNumAdd(A As String, B As String) As String
    Dim i As Integer, j As Integer
    Dim NoI As Integer
    Dim NextNo As Integer
    Dim s As Integer, t As Integer
    Dim Result As String
    Do
        NoI = NoI + 1
        If (Len(A) - NoI) < 0 And (Len(B) - NoI) < 0 Then
            If NextNo > 0 Then
                Result = NextNo & Result
            End If
            Exit Do
        End If
        If (Len(A) - NoI) < 0 Then
            s = 0
        Else
            s = Mid(A, Len(A) - NoI + 1, 1)
        End If
        If (Len(B) - NoI) < 0 Then
            t = 0
        Else
            t = Mid(B, Len(B) - NoI + 1, 1)
        End If
        Dim s_t As Integer
        s_t = s + t + NextNo
        NextNo = s_t \ 10
        Dim S_digits As String
        S_digits = CStr(s_t Mod 10)
        Result = S_digits & Result
    Loop
    BigNumAdd = Result
End Function

最多11个有效位，用double数累加即可，无需拆开累加。

谢谢两位的帮助，试了一下，其实只要Dim a As Double即可。