Data transmission method, device and system

The invention provides a data transmission method, device and system. The method comprises the following steps of: sending a data packet to a receiving device by a sending device, wherein a QoS (Quality of Service) guarantee level is set in the packet head of the data packet; feeding back a confirmation message to the sending device and forwards moving a receiving window by the receiving device when sequentially receiving the data packet according to the QoS guarantee level if the QoS guarantee level belongs to a first level; moving the data packet out of a sending window and forwards moving the sending window by the sending device when receiving the confirmation message; directly moving the data packet out of the receiving window and forwards moving the receiving window by the receiving device after receiving the data packet according to the QoS guarantee level if the QoS guarantee level belongs to a second level; and directly moving the data packet out of the sending window and forwards moving the sending window by the sending device after sending the data packet. According to the invention, the dynamic adjustment requirement on the reliability or time delay of data transmission is realized.