Swelling on one side of the foreskin may be caused by diseases such as prepuce glansitis, allergy, and chronic internal diseases. 1. Foreskin glansitis: it will lead to redness and swelling of the foreskin and glans, accompanied by pain in the foreskin and phimosis. As the foreskin is too long, covering the surface of the glans for a long time, if there is urine retention, or bacteria and other pathogenic microorganisms gathering, leading to the stimulation of the foreskin glans, which will cause redness and swelling of the foreskin and glans. 2. Allergy: the edema of the foreskin and glans may be caused by contact with allergens, mainly edema, usually not accompanied by local redness. 3. Internal medicine chronic diseases: such as heart failure, kidney failure, etc., will cause localized edema of the foreskin, mainly due to low protein in the body or cardiac insufficiency. Swelling on one side of the foreskin may also be seen in the foreskin injury and other diseases, if this symptom continues to be unrelieved, you need to go to the regular hospital in a timely manner to seek medical treatment.