The invention discloses an arm supporting mechanism of a hand wrist joint rehabilitation training device. The arm supporting mechanism comprises an arm bearing plate, wherein a height adjusting deviceis arranged below the arm bearing plate; a supporting plate is arranged between the height adjusting device and the arm beraing plate, and is fixedly connected to an output shaft of the height adjusting device; the arm bearing plate is divided into a first arm bearing plate and a second arm bearing plate; the first and second arm bearing plates are arranged on the supporting plate in parallel, and can slide linearly along the supporting plate to adjust the arm bearing length and height conveniently so as to fit different arm lengths and arm thicknesses of different patients, so that the beraing comfort is improved; the arm bearing mechanism is provided with a positioning beam emitter assisting the arm beraing mechanism to adjust height, so that the arm bearing height can be adjusted conveniently according to assisting beams.