Embodiments of the present invention provide a data transmission method and apparatus. The data transmission method includes: receiving, by a third site, a first transmit frame transmitted by a first site to a second site, and acquiring a transmission end moment of the first transmit frame; transmitting, by the third site, a second transmit frame to a fourth site, and instructing the fourth site to delay returning a response frame; and receiving, by the third site after at least one preset time length after transmission of the second transmit frame ends, the response frame transmitted by the fourth site; where a transmission end moment of the second transmit frame is not later than the transmission end moment of the first transmit frame. The embodiments of the present invention implement that the third site can also transmit data when the first site transmits data, which improves system transmission efficiency.