Knee joint effusion may appear as a dark area of fluid in the knee joint cavity on ultrasound, and the ultrasound is mainly characterized by abnormal echogenicity. If the patient’s condition is at an early stage and the amount of fluid is small, it may appear as anechoic area with good translucency of the fluid. When combined with diseases such as synovitis or periosteal hyperplasia, isoechoic or hypoechoic tissue may be present in the anechoic zone. For knee joint effusions, abnormalities cannot be detected under energy Doppler conditions, and patients mainly present with symptoms such as joint swelling and high skin temperature.