The invention relates to a quantum key distribution method and system based on a tree-shaped QKD network. In a tree network, when a father node of a source node and a father node of a destination nodeare the same node, if the father node is an untrusted node, the source node and the destination node take the father node as an MDI-QKD detector to generate a key, and if the father node is a trustednode, transmission of a shared key is directly carried out through exclusive-OR relay; and when father nodes of the source node and the destination node are not the same node and discontinuous non-trusted relay nodes exist in a transmission path, the non-trusted nodes are taken as an MDI-QKD detector to generate a key, and the shared key is transmitted through an XOR relay method. According to the invention, quantum key distribution between any two points of the tree network under the condition that part of nodes are credible can be realized, and the construction and deployment cost of the quantum key distribution network is effectively reduced.