The pain in the left testicle may be caused by orchitis, epididymitis, varicocele and so on. 1. Testiculitis: the pain in left testicle when it is big and thin can not be ruled out due to the cause of testicular inflammation, testicular inflammation is a disease caused by viral or bacterial infections, after the onset of testicular chills, high fever, swelling, pressure pain, radiating pain and so on. 2. Epididymitis: Epididymitis is a kind of epididymal inflammation caused by pathogenic bacteria entering into the vas deferens, after the onset of the disease, there will be symptoms such as testicles are sometimes big, sometimes thin, swelling, pain, hidden pain, pressure pain, fever and so on. 3. Varicose veins of the spermatic cord: it refers to the blood stagnation of the spermatic cord trailing vascular plexus due to various reasons, resulting in local venous dilatation and tortuosity. The main discomfort is scrotal heaviness and swelling, scrotal pain, hidden pain, sometimes radiating to the ipsilateral groin and perineum. When the appearance of the left testicle when big and thin and pain may have other reasons, it is recommended to actively consult the doctor, under the guidance of a professional physician for diagnosis and treatment.