This invention relates to an elastic protection inversion method and a device to carry out dynamic band width allocation according to the realtime system resources situations in Ethernet passive light network system. When fault happens, the fault circuit and service on key board are inverted to another circuit or card based on different priorities to apply the inversion way of facing ONU and utilize PON label to identify light distribution network to realize protection inversion and bandwidth dynamic redistribution on the harmonic sublayer at the side of OLT and ONU. The device includes an OLT side and ONU side devices connected by ODN, the first is composed of an OLT exchange module, a down-send and an up-receive circuits with protection function, the second includes an ONU exchange module, an up-send and down-receive circuits.