A synchronization method, comprising: sending, by a first device, a first packet to an access device, wherein the first packet carries first timestamp information, and determining second timestamp information, wherein the first timestamp information is a moment that is in an Ethernet network and at which the first device sends the first packet, and the second timestamp information is a moment that is in a mobile network and at which the first device sends the first packet, wherein the mobile network is connected to the Ethernet network; receiving, by the first device, a second packet from the access device, wherein the second packet carries third timestamp information and fourth timestamp information, the third timestamp information is a moment that is in the mobile network and at which a second device receives a third packet sent by the access device to the second device based on the first packet, the fourth timestamp information is a moment that is in the mobile network and at which the second device sends a fourth packet to the access device, and the second packet is sent by the access device to the first device based on the fourth packet, wherein the third packet carries the first timestamp information; determining, by the first device, fifth timestamp information and sixth timestamp information, wherein the fifth timestamp information is a moment that is in the Ethernet network and at which the first device receives the second packet, and the sixth timestamp information is a moment that is in the mobile network and at which the first device receives the second packet; determining, by the first device, seventh timestamp information based on the second timestamp information, the third timestamp information, the fourth timestamp information, the fifth timestamp information, and the sixth timestamp information; and forwarding, by the first device, the seventh timestamp information to the second device by using the access device, wherein the first device is a user plane network element and the second device is a terminal device, or the first device is a terminal device and the second device is a user plane network element. sending, by a first device, a first packet to an access device, wherein the first packet carries first timestamp information, and determining second timestamp information, wherein the first timestamp information is a moment that is in an Ethernet network and at which the first device sends the first packet, and the second timestamp information is a moment that is in a mobile network and at which the first device sends the first packet, wherein the mobile network is connected to the Ethernet network; receiving, by the first device, a second packet from the access device, wherein the second packet carries third timestamp information and fourth timestamp information, the third timestamp information is a moment that is in the mobile network and at which a second device receives a third packet sent by the access device to the second device based on the first packet, the fourth timestamp information is a moment that is in the mobile network and at which the second device sends a fourth packet to the access device, and the second packet is sent by the access device to the first device based on the fourth packet, wherein the third packet carries the first timestamp information; determining, by the first device, fifth timestamp information and sixth timestamp information, wherein the fifth timestamp information is a moment that is in the Ethernet network and at which the first device receives the second packet, and the sixth timestamp information is a moment that is in the mobile network and at which the first device receives the second packet; determining, by the first device, seventh timestamp information based on the second timestamp information, the third timestamp information, the fourth timestamp information, the fifth timestamp information, and the sixth timestamp information; and forwarding, by the first device, the seventh timestamp information to the second device by using the access device, wherein the first device is a user plane network element and the second device is a terminal device, or the first device is a terminal device and the second device is a user plane network element.