The invention relates to a positioning and analyzing method of an oscillation center based on reactive power integral in a multi-machine system. The method comprises the following steps of 1, obtaining dynamic response data of a power grid; 2, extracting the data of a large power grid, so as to obtain the required generator power angle, bus voltage size, bus power and phase angle difference of voltages at the two ends of a connecting line; 3, grouping the system according to the relative power angle of a generator; 4, observing the fluctuation condition of the voltage of a critical branch; 5, carrying out reactive integral on the possible oscillation center; and 6, using the phase angle difference of the bus voltage to detect the obtained branch. The method has the advantages that the cross section of the oscillation center is determined by the reactive power integral of the branch under the assisting of the variation features of electric amount of the oscillation center and the relative power angle difference of the generator, the speed and the precision of the original reactive power integral method are improved, the effect by the running method and the structure change of the power grid is avoided, the method is visual and accurate, the implementation is easy, and the reliability is high.