A method for discovering a PMTU, applicable to a destination node of a path, includes: receiving fragment packets from a source node of the path; determining a detecting range based on a maximum length and a minimum fragment unit of the fragment packets; selecting a detecting value within the detecting range in accordance with a predetermined strategy, requesting the source node to respond with a response packet of a length equal to the detecting value, and determining a PMTU of the path based on whether the response packet from the source node is fragmented.