Why my mouth frequently fills with saliva, also thick phlegm like keep producing from throat any harm swallowin it, why it happens how to get rid f it?

Many causes: see your doctor for a full evaluation. I would consider ruling in/out post-nasal drip, heartburn/acid reflux, chronic allergies or chronic sinusitis, and heart burn/acid reflux. Do you use tobacco? Smoking, a dry environment , not drinking enough water, certain medications can also produce these symptoms. It is safe to swallow mucous, but it may be infected and you should blow your nose.