A method to acquire unbalance of a rotor, for decomposing the unbalance of the rotor and unbalance of a balancing machine; wherein when the balancing machine supports and drives the rotor, the balancing machine and the rotor being measured is in non-rigid connection; the method comprising the following steps setting an angle reference point on the rotor, wherein an angle sensor is installed on the balancing machine; when the angle sensor is on a first position, a plane being formed by the angle sensor and a rotating axis (A1-A2) of the rotor is defined as a first position plane; using the balancing machine to obtain a first measured unbalance of the rotor, the first measured unbalance is represented in a measuring plane 1 perpendicular to the rotating axis (A1-A2); the method is characterized by further comprising the following steps: moving the angle sensor from the first position to a second position on the balancing machine; when the angle sensor is on the second position, a plane being formed by the angle sensor and the rotating axis (A1-A2) of the rotor is defined as a second position plane; an included angle (α) is formed between the second position plane and the first position plane, and the second position plane is in an opposite direction of rotor rotation relative to the first position plane; using the balancing machine to obtain a second measured unbalance of the rotor in the measuring plane 1; using the first measured unbalance and the second measured unbalance, acquiring, by vector calculation, a calculated unbalance of the rotor; the vector calculation comprises: forming an isosceles triangle by taking a line connecting a vector end of the first measured unbalance and a vector end of the second measured unbalance as a base side, and the included angle α as a vertex angle; wherein, from isosceles side of the vertex to the vector end of the first measured balance to isosceles side of the vertex to the vector end of the second measured balance is clockwise, the vector from the vertex of the isosceles triangle to the vector end of the first measured unbalance is the calculated unbalance of the rotor. the method comprising the following steps setting an angle reference point on the rotor, wherein an angle sensor is installed on the balancing machine; when the angle sensor is on a first position, a plane being formed by the angle sensor and a rotating axis (A1-A2) of the rotor is defined as a first position plane; using the balancing machine to obtain a first measured unbalance of the rotor, the first measured unbalance is represented in a measuring plane 1 perpendicular to the rotating axis (A1-A2); the method is characterized by further comprising the following steps: moving the angle sensor from the first position to a second position on the balancing machine; when the angle sensor is on the second position, a plane being formed by the angle sensor and the rotating axis (A1-A2) of the rotor is defined as a second position plane; an included angle (α) is formed between the second position plane and the first position plane, and the second position plane is in an opposite direction of rotor rotation relative to the first position plane; using the balancing machine to obtain a second measured unbalance of the rotor in the measuring plane 1; using the first measured unbalance and the second measured unbalance, acquiring, by vector calculation, a calculated unbalance of the rotor; the vector calculation comprises: forming an isosceles triangle by taking a line connecting a vector end of the first measured unbalance and a vector end of the second measured unbalance as a base side, and the included angle α as a vertex angle; wherein, from isosceles side of the vertex to the vector end of the first measured balance to isosceles side of the vertex to the vector end of the second measured balance is clockwise, the vector from the vertex of the isosceles triangle to the vector end of the first measured unbalance is the calculated unbalance of the rotor.