Three and a half year old baby boy always want to pee, may be too much water, mental urinary frequency, urinary tract infection disease, diabetes and other factors. 1. drinking too much water: three and a half year old baby drinking too much water, and too little activity, the amount of urine generated more frequent urination, reduce the amount of drinking water can return to normal. 2. mental urinary frequency: three and a half year old baby boy encountered some unexpected things, mental tension or fear, easy to induce mental urinary frequency, there may be frequent urination. 3. urinary tract infection disease: do not pay attention to lower body hygiene, may induce urinary tract infection disease, such as urethritis or cystitis, so that the urethra or bladder mucous membrane sensitivity is too high, there will be frequent urination, also accompanied by urinary urgency, urinary pain and other symptoms. 4. Diabetes: may be related to genetic, environmental factors, viral infections and other factors, resulting in abnormal blood sugar in the body, it will be frequent urination, accompanied by drinking, eating, weight loss, fatigue and other symptoms. Three and a half year old baby boy always want to urinate, and accompanied by obvious other clinical symptoms, it is recommended to seek medical examination and treatment.