The present invention improves a conventional low-frequency oscillation centralized operational mode into a distributed operational mode, and discloses a low-frequency oscillation mode layered detection method for a power system noise signal, i.e. a low-frequency oscillation monitoring function is deployed on a substation layer and a dispatching center layer. On the substation layer, PMU data of a substation is utilized to carry out local low-frequency oscillation detection and frequency spectrum estimation, and on the dispatching center layer, synthetic estimation is carried out according to low-frequency oscillation result data sent by the substation layer. In order to achieve synthesis of low-frequency oscillation detection results of the substation layer on the dispatching center layer, firstly, all the PMU data of all systems are identified, then a least squares support vector machine (LSSVM) is adopted to carry out fitting with the low-frequency oscillation detection result of the substation layer, and finally, all the systems can be subjected to rapid synthetic judgment only according to the low-frequency oscillation results uploaded by the substation so as to determine an interval oscillation mode and a local oscillation mode of the whole system.