A branch pruning mechanical arm consists of a base, a bottom cover, a rotary table, a big bevel gear, a small bevel gear, a motor, a vertical seat, a lifting oil cylinder, a lower arm, a flexible arm, a rotary head, a tilt angle oil cylinder, a disc saw flexible oil cylinder, a mechanical claw driving oil cylinder, a mechanical claw, a disc saw, a hydraulic motor and the like. The rotary table is driven by the motor to rotate by 360 degrees. The lifting oil cylinder is controlled to stretch, so that the mechanical arm can stand up. The flexible arm is driven by a traction wire rope or a gear rock to extend or retract node by node. A pruning mechanism can rotate by 360 degrees around the axis thereof by controlling the working head tilt angle oil cylinder and the rotary head. By controlling the oil cylinder to retract, the mechanical claw reliably clamps a to-be-pruned branch. The hydraulic motor drives the disc saw to rotate at a high speed, and controls the flexible oil cylinder to extend out at the same time, so that the disc saw gradually cuts off the to-be-pruned branch. By controlling the oil cylinder, the flexible arm and the rotary table, the pruned branches are reliably conveyed back to the ground, so that the branches are pruned.