This application describes a packet flow out-of-order detection method. After receiving a first packet from a second node, a first node parses the first packet to obtain a flow segment sequence number of the first packet. After receiving the first packet, the first node continues to receive a second packet from the second node, and parses the second packet to obtain a flow segment sequence number of the second packet. The first packet and the second packet belong to a same packet flow. In view of this, the first node may compare the flow segment sequence number of the first packet with the flow segment sequence number of the second packet, to determine whether the packet flow is out of order.