The invention provides a data transmission method and device, and relates to the field of computers. The data transmission method comprises the steps that jittering of the round-trip delay of a transmission message sent by a sending end is acquired, and whether or not jittering of the round-trip delay of the transmission message is higher than a congestion judging threshold is judged, wherein jittering of the round-trip delay represents the range that the round-trip delay of the transmission message deviates from the standard round-trip delay; if jittering of the round-trip delay is higher than the congestion judging threshold, the current bandwidth is acquired, and whether or not the decreasing range of the current bandwidth exceeds a decreasing threshold is judged; and if the current bandwidth is not decreased or the decreasing range does not exceed the decreasing threshold, a congestion window is narrowed according to a first preset microstep. According to the technical scheme, theutilization rate of the network bandwidth is guaranteed maximally on the basis that the influences brought by random packet loss are relieved.