The invention provides a robot flexible joint with continuously adjustable stiffness and relates to the field of robots. The robot flexible joint with continuously adjustable stiffness comprises a driving end, a buffering mechanism and an output flange as well as a stiffness adjusting mechanism, wherein the driving end is used for providing main power for a joint, the buffering mechanism is connected between the driving end and the output flange, used for transmitting the main power of the driving end to the output flange and comprises an elastic element; and the stiffness adjusting mechanism moves and is connected with the buffering mechanism and changes pre-compression amount of the elastic element by moving toward the buffering mechanism, and stiffness adjustment on the joint is realized. The robot joint provided by the invention has the advantages that the stiffness is continuous and adjustable, the flexible joint can effectively reduce an external impact force, and the security of the robot and environmental friendliness are improved.