A packet sending method is applied to a data transmission system and comprising: receiving, by a network interface card of a second device, a first packet that is from a first QP of a first device and that is forwarded by using a switch; obtaining, by the network interface card of the second device, an instant rate of the first QP and a maximum sending rate of the first QP; determining, by the network interface card of the second device, an adjustment rate of the first QP based on the instant rate of the first QP and the maximum sending rate of the first QP; sending, by the network interface card of the second device, a congestion notification packet to the first device, where the congestion notification packet carries the adjustment rate of the first QP.