A packet loss detecting method includes: a first router counts received packets and sent packets respectively according to a class of service value of a packet in each tunnel; the first router generates a first packet loss detection packet corresponding to the class of service value, sends the first packet loss detection packet to a second router at a peer end, and receives a first packet loss response packet returned by the second router; and the first router corresponds a class of service value, and measures a packet loss value according to a locally recorded count value and a count value carried in the first packet loss response packet. With the technical solution, packet disorder does not occur when packet loss measurement is performed for a pseudo-wire with multiple classes of service. In addition, traffic detection occupies a smaller bandwidth.