One or more embodiments of this specification provide block synchronization methods and apparatuses. A method includes: receiving, by a block synchronization device in a blockchain relay communication network and from a relay node of a plurality of relay nodes, a block obtaining request sent by a blockchain node of a plurality of blockchain nodes; selecting, by the block synchronization device from one or more locally stored blocks on the block synchronization device, one or more historical blocks; and sending, by the block synchronization device via the relay node of the plurality of relay nodes, one or more historical blocks to the blockchain node of the one or more blockchain nodes to perform block synchronization by the blockchain node of the one or more blockchain nodes on blocks locally stored by the blockchain node of the plurality of blockchain nodes based on the one or more historical blocks.