您可以使用NumberFormatInfo.NumberGroupSeparator 属性
来自 MSDN 的示例
using System;
using System.Globalization;
class NumberFormatInfoSample {
    public static void Main() {
    // Gets a NumberFormatInfo associated with the en-US culture.
    NumberFormatInfo nfi = new CultureInfo( "en-US", false ).NumberFormat;
    // Displays a value with the default separator (",").
    Int64 myInt = 123456789;
    Console.WriteLine( myInt.ToString( "N", nfi ) );
    // Displays the same value with a blank as the separator.
    nfi.NumberGroupSeparator = " ";
    Console.WriteLine( myInt.ToString( "N", nfi ) );
    }
}
/* 
This code produces the following output.
123,456,789.00
123 456 789.00
*/
为您 - 将 NumberGroupSeparator 属性设置为 '/'
更新
另一个样本
var t = long.Parse("123/456/789",NumberStyles.Any, new NumberFormatInfo() { NumberGroupSeparator = "/" });
var output = string.Format(new NumberFormatInfo() { NumberGroupSeparator="/"}, "{0:0,0}", t);