The hydraulic ankle joint includes a main member having a fluid supply chamber storing a hydraulic fluid, a first pressure regulating chamber, a second pressure regulating chamber, and a space connecting the chambers. The space houses an axle element rotatably joined to the main member. The main member is rotatably mounted on a U-shaped hinge seat. An extension piece is extended radially from the circumference of the axle element. A fluid block penetrates through the main member and into the space. The extension piece and the fluid block jointly partition the space into a front chamber and a back chamber. The first and second pressure regulating chambers adjust the back and front chambers pressure so that impact from walking may be effectively buffered. The fluid supply chamber automatically refills hydraulic fluid into the front and back chambers so as to reduce user maintenance overhead.
