The invention discloses a data backup method and device and a distributed file system. The data backup method comprises, when a data backup request transmitted by a data node is received, transmitting a query request to a cost server for querying cost matrix information in a distributed system, which is associated with the data node; receiving response information transmitted by the cost server, wherein the cost information comprises the cost matrix information associated with the data node; computing the backup cost between the data node and any other candidate node through the cost matrix information, selecting the candidate node with the lowest backup cost as a target node, and transmitting the information of the target node to the data node so as to back data up to the target node through the data node. According to the data backup method, data backup is performed by selecting the node with the lowest backup cost according to a cost matrix, so that the problem of overhigh backup cost and node load non-uniformity can be effectively avoided.