In a data transmission method, a first communication device obtains a data packet in a first direction, and obtains a data packet in a second direction. The two data packet correspond to a same service. If the first direction is an uplink direction, the second direction is a downlink direction; or if the first direction is a downlink direction, the second direction is an uplink direction. The first communication device determines a target transmission delay in the second direction based on a total transmission delay requirement and a time point at which the data packet in the first direction is sent. The first communication device sends the data packet in the second direction based on the target transmission delay in the second direction. The total transmission delay requirement is a sum of a transmission delay requirement in the first direction and a transmission delay requirement in the second direction.