A method to measure and determine the unbalance of a rotor, by decoupling the unbalance of the balancing machine and the unbalance of the rotor, which includes the following procedures make the zero angle reference on the rotor or on the balancing machine, select two planes on the rotor, and use a balancing machine to measure the unbalances of the rotor in the two planes, these unbalances are designated as first nominal unbalances.