A brake assembly for a robot joint, the brake assembly comprises: a stator ring, a channel being arranged in the stator ring, the channel containing hydraulic oil, the channel comprising a first channel section and a second channel section communicating with each other through a communication channel, switch parts (111, 115) being arranged in the communication channel and being configured to open and close the communication channel, a plurality of first holes extending radially being opened in the radial inner surface of the stator ring in the first channel section, a plurality of first brake parts being airtightly arranged in the corresponding first holes and being configured to move between a stretching position and a retraction position along the axes of the corresponding first holes, a plurality of second holes extending radially being opened in the radial inner surface of the stator ring in the second channel section, a plurality of second brake parts being airtightly arranged in the corresponding second holes and being configured to move between a stretching position and a retraction position along the axes of the corresponding second holes, and a rotor disc concentrically arranged in the stator ring, the rotor disc being configured to rotate around the axis common to the stator ring relative to the stator ring, the outer circumference of the rotor disc having a curved tooth profile, wherein, the pressure of the hydraulic oil is configured to that each first brake part and each second brake part simultaneously contact the curved tooth profile of the rotor disc, the positions of the plurality of first holes and the plurality of second holes and the curved tooth profile are configured to that the sum of the displacements of all the first brake parts and all the second brake parts relative to the retraction position is constant during a rotation of the rotor disc and thus the rotor disc is configured to rotate relative to the stator ring when the switch parts (111, 115) do not close the communication channel, and in addition, the positions of the plurality of first holes and the curved tooth profile are configured to that the sum of the displacements of all the first brake parts relative to the retraction position is not constant during a rotation of the rotor disc and thus the first brake parts and the second brake parts are configured to brake the rotor disc when the switch parts (111, 115) close the communication channel. a stator ring, a channel being arranged in the stator ring, the channel containing hydraulic oil, the channel comprising a first channel section and a second channel section communicating with each other through a communication channel, switch parts (111, 115) being arranged in the communication channel and being configured to open and close the communication channel, a plurality of first holes extending radially being opened in the radial inner surface of the stator ring in the first channel section, a plurality of first brake parts being airtightly arranged in the corresponding first holes and being configured to move between a stretching position and a retraction position along the axes of the corresponding first holes, a plurality of second holes extending radially being opened in the radial inner surface of the stator ring in the second channel section, a plurality of second brake parts being airtightly arranged in the corresponding second holes and being configured to move between a stretching position and a retraction position along the axes of the corresponding second holes, and a rotor disc concentrically arranged in the stator ring, the rotor disc being configured to rotate around the axis common to the stator ring relative to the stator ring, the outer circumference of the rotor disc having a curved tooth profile, wherein, the pressure of the hydraulic oil is configured to that each first brake part and each second brake part simultaneously contact the curved tooth profile of the rotor disc, the positions of the plurality of first holes and the plurality of second holes and the curved tooth profile are configured to that the sum of the displacements of all the first brake parts and all the second brake parts relative to the retraction position is constant during a rotation of the rotor disc and thus the rotor disc is configured to rotate relative to the stator ring when the switch parts (111, 115) do not close the communication channel, and in addition, the positions of the plurality of first holes and the curved tooth profile are configured to that the sum of the displacements of all the first brake parts relative to the retraction position is not constant during a rotation of the rotor disc and thus the first brake parts and the second brake parts are configured to brake the rotor disc when the switch parts (111, 115) close the communication channel.