Thanks.
Is MTF(each TF is used only one indicator) also in the danger of overfitting?
Sure thing. Just think in terms of degrees of freedom, as jcl nicely put. The first degree of freedom is what is your first TF? The second one is your second TF, and so forth... Then when you come to indicators, say you're using MACD. It has three degrees of freedom by itself, because of three parameters it takes to be exactly defined (12, 26, 9). Apply accordingly to others indicators also. Soon you'll see that you have so many parameters to choose from, that your search for a holy grail has quite some chance to succeed. Unfortunately, it will be so perfect only for the time period of your backtest, aka past time. But will fail miserably going forward, out of sample. Obviously, you overfitted. Or said in other words, you've been fooled by randomness.
There's one technique that can help you to avoid overfit, and that is to carefully check your parameter space after optimization. For example, once I had a "great" trading strategy with these parameters: 17, 9, 1. Backtested over 5 years, it gave excellent results. But, as soon as I changed 17 to 18, it would fail. Obviously, not a good trading system. Your final parameters should allow for quite some tolerance, if you expect the trading system to be robust out of sample.
One other way to attack this issue is to use discretized values, so to skip large parts of parameters space and thus lower the chance of overfit. For example, if you're tuning the period of moving average between 50 and 200, don't use all values, but go in steps of say 5, like 75, 80, 85, etc... But also apply the principle from my last paragraph, if you're doing great with SMA80, but results are bad with SMA75, then you probably overfitted.
Back to your example of MTF. Say you're going to use 5 timeframes. That's 5 parameters that can produce a very large number of possible combinations. You're practically guaranteed you will find at least one combination which will produce excellent results. Do you really think it would survive the test of time?
I hope my english is good enough to attempt to explain these very important concepts in automated trading.