The utility model discloses a mechanical arm for a pecan picking robot. The mechanical arm comprises a swing mechanism, wherein the swing mechanism comprises a swing motor, a swing motor frame, a support plate and a bearing plate; the bearing plate is fixed above the support plate through four support posts; the swing motor is fixed on the swing motor frame; the swing motor frame is fixed on the support plate; an output shaft of the swing motor is connected with a connecting shaft; the connecting shaft penetrates through the bearing plate; a horizontal bearing and a horizontal support are fixed on the bearing plate; the connecting shaft is fixedly connected with the horizontal bearing and the horizontal support; the horizontal support is connected with a base plate; the base plate is fixedly provided with a left row of vertical bearings and a right row of vertical bearings; a left main arm is connected with the vertical bearings through a basic shaft; a right support arm is connected with the vertical bearings through two disconnected half shafts; and the main arm and the support arm are connected through a pin. The mechanical arm is simple in structure, high in operability, accurate in positioning and smooth in operation, can free pecan picking workers from heavy work, and greatly reduces pecan picking risks.