Flat nails may be caused by trauma, psoriasis, onychomycosis, and so on. 1. Trauma: it may be due to nails being extruded by external force or impacted by heavy objects, leading to the current appearance of flat nails, often accompanied by obvious pain. 2. Psoriasis: It may be caused by infection, trauma, mental stress, etc. If psoriasis involves the nails, it may lead to thinning or thickening of the nails. 3. Onychomycosis: also known as gray nails, it is mainly due to the lesions caused by the infection of the nail bed by the skin fungus, which often leads to flat nails, yellowing, thickening and so on. Flat nails may also be caused by other reasons, need to go to the hospital in time to find out the cause of targeted treatment.