There are many clinical causes of pain on the left side of the knee when pressed, commonly due to lateral collateral ligament injury, bursitis, and meniscal injury, as follows: 1) lateral collateral ligament injury of the knee, due to late rest and protection without strict attention, the injury is not fully recovered, and local sterile inflammation is formed, and pain occurs when pressed; 2) bursitis, mostly due to local sterile inflammation of the bursa caused by recent strain or cold; 3) meniscal injury can also cause pain when pressed, and usually requires ancillary tests to clarify the diagnosis, such as MRI of the knee.