Traditional Chinese Medicine (TCM) theory reasons that the deficiency of healthy Qi and attack of pathogenic wind, cold and damp factors on the meridians of the limbs induce the Bi-syndrome, a syndrome marked by arthragia, numbness and dyskinesia of the limbs. As for the Pyretic Bi, it's due to invasion of joints by toxious heat or by accumulated heat and further complicated by pathogenic wind, cold and dampness, characterized with pain, redness, heat and swelling of the joint, thirst and fever. This case introduced a 50 years old female suffered from arthragia that caused by wind-cold-damp pathogenic factors in the past. Her Yin-deficiency constitution and overstrain of the knee muscle and tendon further caused the infrapatellar bursitis. TCM diagnosis of this disease is acute Pyretic Bi with heat predominated. After treatment with heat and toxic clearing and diuretic herbal formula for one week, the patient recovered completely. This article further discusses the etiology and the treatment of Pyretic Bi, and also the differential diagnosis with similar arthralgia or tendonitis around knee joint in modern medicine.