.Net <= 4.0

TimeSpan t = TimeSpan.FromSeconds( secs );

string answer = string.Format("{0:D2}h:{1:D2}m:{2:D2}s:{3:D3}ms", 
                t.Hours, 
                t.Minutes, 
                t.Seconds, 
                t.Milliseconds);

.NET > 4.0

TimeSpan time = TimeSpan.FromSeconds(seconds);
string str = time .ToString(@"hh\:mm\:ss\:fff");

参考：
https://stackoverflow.org.cn/questions/463642