A computer-implemented method for implementing blockchain-based private transactions, the method comprising: transmitting, by a first blockchain node of a number N of blockchain nodes that are involved in a private transaction, the private transaction to each of a plurality of second blockchain nodes of the N blockchain nodes, wherein N is at least four; obtaining, by the first blockchain node, at least N-F signatures certifying the private transaction respectively from at least N-F blockchain nodes of the first and second blockchain nodes, wherein F is a largest integer no more than (N-1)/2; submitting, by the first blockchain node, a blockchain transaction comprising a hash value of the private transaction in association with the at least N-F signatures, the at least N-F signatures, and a group identification corresponding the N blockchain nodes to one or more blockchain nodes of a public blockchain for adding into the public blockchain, wherein the group identification, when stored into the public blockchain, serves as a notification to the N blockchain nodes of a successful consensus verification of the private transaction; obtaining, by the first blockchain node, the private transaction from the public blockchain; verifying, by the first blockchain node, the private transaction by verifying if a number of signatures comprised in the obtained private transaction reaches at least N-F; upon the verification being successful, locally executing, by the first blockchain node, the private transaction according to an order of the private transaction stored in the public blockchain relative to other private and public transactions stored in the public blockchain; receiving, by the first blockchain node, a synchronization request from a requesting blockchain node; determining, by the first blockchain node, if the requesting blockchain node is one of the N blockchain nodes; and in response to determining that the requesting blockchain node is one of the N blockchain nodes, transmitting, by the first blockchain node, the private transaction to the requesting blockchain node via a non-blockchain channel. transmitting, by a first blockchain node of a number N of blockchain nodes that are involved in a private transaction, the private transaction to each of a plurality of second blockchain nodes of the N blockchain nodes, wherein N is at least four; obtaining, by the first blockchain node, at least N-F signatures certifying the private transaction respectively from at least N-F blockchain nodes of the first and second blockchain nodes, wherein F is a largest integer no more than (N-1)/2; submitting, by the first blockchain node, a blockchain transaction comprising a hash value of the private transaction in association with the at least N-F signatures, the at least N-F signatures, and a group identification corresponding the N blockchain nodes to one or more blockchain nodes of a public blockchain for adding into the public blockchain, wherein the group identification, when stored into the public blockchain, serves as a notification to the N blockchain nodes of a successful consensus verification of the private transaction; obtaining, by the first blockchain node, the private transaction from the public blockchain; verifying, by the first blockchain node, the private transaction by verifying if a number of signatures comprised in the obtained private transaction reaches at least N-F; upon the verification being successful, locally executing, by the first blockchain node, the private transaction according to an order of the private transaction stored in the public blockchain relative to other private and public transactions stored in the public blockchain; receiving, by the first blockchain node, a synchronization request from a requesting blockchain node; determining, by the first blockchain node, if the requesting blockchain node is one of the N blockchain nodes; and in response to determining that the requesting blockchain node is one of the N blockchain nodes, transmitting, by the first blockchain node, the private transaction to the requesting blockchain node via a non-blockchain channel.