Abstract: With the rapid development of UHV AC/DC projects, the power system is vulnerable to instability of multi-group modes. As a result, it is urgent to research the migration rule of the out-of-step center in multiple-frequency conditions. Based on theoretical derivation of the swing center location in the three machines system, the migration rule of the swing center in multi-frequency conditions was analyzed. Accordingly, the necessary and sufficient condition for the out-of-step center was obtained, which was depicted in a voltage phase angle plane where the phase angle of three machines should satisfy the constraints. Based on this, a pre-warning strategy of the out-of-step center was proposed. According to the voltage angle and frequency as the phase angle trajectory approaches to the constraints, the appearance time and location of the out-of-step center can be calculated. So the splitting device is activated in advance, which guarantees it works in time. Finally, IEEE 9 and IEEE 118 system in PSS/E verify the effectiveness of the forecasting method.