The invention discloses a method for automatically compensating the density and an angle of a full-automatic motor rotor balancing machine, which comprises the following steps: in a measuring station, after being found out and measured, a reference point of a processed motor rotor is downward; the motor rotor is grasped to a cutting station by a manipulator to finish the cutting of both surfaces and clamped by a jig turning device to continuously rotate to the reference point and is just downward; the rotor is grasped to a re-measurement station by the manipulator to re-measure; the re-measurement station judges whether the processed motor rotor is qualified or not according to the residual unbalance of re-measurement; and if the processed motor rotor is qualified, no processing is carried out, or else the equivalent density of an iron core and the angle of a sensor are compensated according to the residual unbalance. When the residual unbalance re-measured at both processing surfacesis in one positive and negative gear range of the front surface and the reverse surface of a vector of initial unbalance, the weighting compensation is carried out on the equivalent density; when theresidual unbalance re-measured at both processing surfaces is in the homodromous rotating direction of the front surface of the vector of the initial unbalance, the weighting compensation is carried out on the angle of the sensor.