The invention relates to a flexible humanoid knee joint, in particular to a flexibility-adjustable humanoid knee joint. The problems that most knee joints of existing traditional humanoid robots are articulated single-shaft joints, the rotating centers are fixed, motors and reducers are mostly adopted as driving devices, flexibility lacks, and impact of the outside on the knee joints cannot be buffered in the moving process can be solved. The flexibility-adjustable humanoid knee joint comprises a driving mechanism and a transmission mechanism. The driving mechanism comprises a transmission wheel, a metal rope, a first rotating shaft and two pneumatic artificial muscles. The transmission mechanism comprises a central gear, a planetary gear, a tie bar and a second rotating shaft. The upper ends of the two pneumatic artificial muscles are connected with a thigh; the central gear is fixedly mounted on the thigh, the planetary gear is fixedly mounted on a shank, and the central gear and the planetary gear are engaged; the first rotating shaft is connected with the second rotating shaft through the tie bar; the transmission wheel is fixedly mounted on the first rotating shaft, and the metal rope is wound on the transmission wheel. The flexibility-adjustable humanoid knee joint is applied to a humanoid robot.