A network performance monitoring method, comprising: receiving, by a network performance monitoring apparatus, a second packet sent by a first network device, wherein the second packet is a duplicate packet of a first packet, the second packet is copied when the first network device sends the first packet to a second network device, a destination receive end of the first packet is the second network device, a type of the first packet is a packet of a traffic flow transmitted by using a connection-oriented protocol or a packet that is used to establish a connection, and the connection is used to transmit a traffic flow by using a connection-oriented protocol; recording, by the network performance monitoring apparatus, a first time, wherein the first time is a time at which the network performance monitoring apparatus receives the second packet; receiving, by the network performance monitoring apparatus, a fourth packet sent by the first network device, wherein the fourth packet is a duplicate packet of a third packet, the fourth packet is copied when the first network device receives the third packet sent by the second network device, a type of the third packet is the same as the type of the first packet, and a source transmit end of the third packet is the second network device; when the network performance monitoring apparatus determines that the fourth packet is a response packet of the second packet, recording, by the network performance monitoring apparatus, a second time, wherein the second time is a time at which the network performance monitoring apparatus receives the fourth packet; and determining, by the network performance monitoring apparatus, a round-trip delay between the first network device and the second network device according to the first time and the second time; wherein if the network performance monitoring apparatus is disposed outside the first network device, the receiving, by the network performance monitoring apparatus, the second packet sent by the first network device comprises: receiving, by the network performance monitoring apparatus, the second packet obtained by the first network device by copying the first packet in a mirroring manner; and the receiving, by the network performance monitoring apparatus, the fourth packet sent by the first network device comprises: receiving, by the network performance monitoring apparatus, the fourth packet obtained by the first network device by copying the third packet in the mirroring manner. receiving, by a network performance monitoring apparatus, a second packet sent by a first network device, wherein the second packet is a duplicate packet of a first packet, the second packet is copied when the first network device sends the first packet to a second network device, a destination receive end of the first packet is the second network device, a type of the first packet is a packet of a traffic flow transmitted by using a connection-oriented protocol or a packet that is used to establish a connection, and the connection is used to transmit a traffic flow by using a connection-oriented protocol; recording, by the network performance monitoring apparatus, a first time, wherein the first time is a time at which the network performance monitoring apparatus receives the second packet; receiving, by the network performance monitoring apparatus, a fourth packet sent by the first network device, wherein the fourth packet is a duplicate packet of a third packet, the fourth packet is copied when the first network device receives the third packet sent by the second network device, a type of the third packet is the same as the type of the first packet, and a source transmit end of the third packet is the second network device; when the network performance monitoring apparatus determines that the fourth packet is a response packet of the second packet, recording, by the network performance monitoring apparatus, a second time, wherein the second time is a time at which the network performance monitoring apparatus receives the fourth packet; and determining, by the network performance monitoring apparatus, a round-trip delay between the first network device and the second network device according to the first time and the second time; wherein if the network performance monitoring apparatus is disposed outside the first network device, the receiving, by the network performance monitoring apparatus, the second packet sent by the first network device comprises: receiving, by the network performance monitoring apparatus, the second packet obtained by the first network device by copying the first packet in a mirroring manner; and the receiving, by the network performance monitoring apparatus, the fourth packet sent by the first network device comprises: receiving, by the network performance monitoring apparatus, the fourth packet obtained by the first network device by copying the third packet in the mirroring manner. receiving, by the network performance monitoring apparatus, the second packet obtained by the first network device by copying the first packet in a mirroring manner; and the receiving, by the network performance monitoring apparatus, the fourth packet sent by the first network device comprises: receiving, by the network performance monitoring apparatus, the fourth packet obtained by the first network device by copying the third packet in the mirroring manner.