A method for accessing data across a multi-chain system comprising multiple blockchains, the method comprising: receiving, from a node device of a blockchain in the multi-chain system, a request for access to target data stored by a target blockchain, wherein the request comprises an access path of the target data, the access path indicating the target blockchain that stores the target data, wherein the access path is in a data format of a multi-layer structure and comprises i) a data node domain field used to record area information about an area that a start data node accessing the target data is located in, the area information comprising identification information of a target blockchain that the start data node is located in, wherein the data node domain field comprises a retrieval space field used to record identification information of a data retrieval space used when identification information of the start data node is used as a query index to query the target data on the target blockchain, ii) a data node identifier field used to record identification information of the start data node, and iii) a data node path field used to record access path information of the target data traversed and identified starting from the start data node; obtaining, based on the access path, the target data from the target blockchain; and returning the target data to the node device. receiving, from a node device of a blockchain in the multi-chain system, a request for access to target data stored by a target blockchain, wherein the request comprises an access path of the target data, the access path indicating the target blockchain that stores the target data, wherein the access path is in a data format of a multi-layer structure and comprises i) a data node domain field used to record area information about an area that a start data node accessing the target data is located in, the area information comprising identification information of a target blockchain that the start data node is located in, wherein the data node domain field comprises a retrieval space field used to record identification information of a data retrieval space used when identification information of the start data node is used as a query index to query the target data on the target blockchain, ii) a data node identifier field used to record identification information of the start data node, and iii) a data node path field used to record access path information of the target data traversed and identified starting from the start data node; i) a data node domain field used to record area information about an area that a start data node accessing the target data is located in, the area information comprising identification information of a target blockchain that the start data node is located in, wherein the data node domain field comprises a retrieval space field used to record identification information of a data retrieval space used when identification information of the start data node is used as a query index to query the target data on the target blockchain, ii) a data node identifier field used to record identification information of the start data node, and iii) a data node path field used to record access path information of the target data traversed and identified starting from the start data node; obtaining, based on the access path, the target data from the target blockchain; and returning the target data to the node device.