The present invention provides an Ethernet performance detection method and system and an optical network terminal. The method includes: receiving, by an ONT, a detection configuration instruction, and configuring a maintenance end point according to the detection configuration instruction; configuring a performance detection path from the MEP to the VMEP according to states of the virtual maintenance end points configured on a main node and a backup node, wherein the VMEP includes two virtual MEPs, the IDs of the two virtual MEPs are identical, at the same moment; and transmitting a message to a node corresponding to the virtual MEP with the main state. When the node states of the main node and backup node are switched, the optical network terminal may automatically switch the performance detection path to the node corresponding to the virtual MEP, in order to continue to perform the network performance detection.