One of my sites has CTR 18%, is that too much high ? Can I have some problems with google because of it ?
It is on the high side, yes. A good CTR on a well optimised site is about 5-10% (closer to 5%). I'd check through your visit logs and see if there's anything suspicious. Google will have already noticed this high rate. It is probably safest to report this unusually high CTR to Google - just tell them you're concerned it is a bit high and ask them to assess the clicks.
I think it depends on your site and your traffic. If you have 18% CTR on forum site, that seems really high. If you have a content site with well targeted ads, I don't think 18% is at all unreasonable. It will also depend on where you get your traffic. My experience is SE traffic doesn't click as often as PPC traffic. People who click ads to get to your site are more likely to click ads on your site. You might consider investing time or money in some Adsense tracking software. I believe there are some free scripts that are available. For myself, I purchased Adsense Tracker. It isn't perfect, but I think it gives you a lot of good information. That may help you determine if it is one person clicking one ad 100 times (a problem) or 100 different people clicking 50 different ads (no problem). Tracking software will also allow you to understand your website traffic so you will notice when something is not quite right. With the concern of being banned from Adsense hanging over everyone's head, knowing what is going on with your site helps you sleep at night.