i have a blog which received many comments. but most are linked with casino keyword is it harmful for adsense please help me. thanks.
Those are spam messages. I receive tons of them and I never approve such mess. I'm sure it will affect your overall website quality.
Blogs are unfortunately a magnet for spam. If your comments are dofollow then it's a never ending job cleaning up the spam comments, unfortunately.
Your site isn't allowed to promote or link to online casinos if you are also displaying AdSense ads. Spam comments linking to sites like that (or viagra or porn or whatever) can get your AdSense account into trouble. Just remove them.