A swollen knee may be caused by bursitis, knee trauma, mosquito bites, and the cause needs to be clarified. 1. Bursitis: due to repeated long-term friction, compression of the bursa or infection, which leads to inflammation, can lead to swelling of the bursa, found that the knee swollen up a big bag. There will also be joint stiffness, pain, skin redness and swelling and other symptoms. 2. Knee trauma: when the knee is bumped and traumatized, it can cause local capillary rupture and bleeding, which can also result in swelling of the knee. It can also be accompanied by knee pain, restricted movement and other symptoms. 3. Mosquito bites: the knee area by mosquito bites, the knee will also be swollen a big bag. There may be other reasons for the swelling of the knee, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, give targeted treatment or treatment.