The invention provides a data transmission method and a system in P2P live streaming. The method comprises the following steps that: video data is divided into video sub-blocks, and the video sub-blocks are distributed to the nodes of a sender to be transmitted; the nodes of the sender send the obtained video sub-blocks to the nodes of a receiver, and the missing video sub-blocks are positioned; the routing to the candidate nodes is calculated, and the candidate nodes comprise the missing video sub-blocks; the video sub-blocks sent by the nodes of the sender are received, the candidate nodes with the least sub-nodes are selected to be connected according to the routing information, and the missing video sub-blocks are preset; and when the candidate nodes receive the missing video sub-blocks, the video sub-blocks are transmitted to the nodes of the receiver. The invention reduces the delayed time for a terminal user to watch the video, improves the real time property of a live system, effectively reduces the control expenditure of the system, and greatly improves the throughput of the system.