The invention discloses a three-segment out-of-step self-adaptive disconnecting method. The method utilizes effective rapid detection to judge system out-of-step and accurately measure criterions of an oscillation center, and performs disconnecting judgment in three segments according to whether the oscillation center is in a protection scope of a device-installing position during the out-of-step, thus an out-of-step disconnecting device can perform self-adaptive judgment for disconnection as a relay protection device so as to provide convenience for setting fixed values of users and adapt to the changes of system modes. Therefore, the out-of-step disconnecting device as a third defense line of a power system can rapidly and accurately act so as to guarantee the safe steady operation of the power system.