The invention discloses a method and device for data block migration. The method includes the steps that the maximum bandwidth which permits an I/O request of an upper-layer application on a bottom-layer hierarchical storage device is set, flow of the I/O request of the upper-layer application on the bottom-layer hierarchical storage device is monitored, remaining bandwidth of the bottom-layer hierarchical storage device is calculated, when a data block needs to be migrated, whether the remaining bandwidth of the bottom-layer hierarchical storage device exists is judged, and if yes, migration I/O operation of the data block is executed. According to the method and device, under the condition that intensive migration I/O operation occurs, sufficient bandwidth can be supplied for the migration I/O operation. The remaining bandwidth is used for executing the migration I/O operation of data blocks needing migration, batch data migration is achieved, real-time performance and migration efficiency of data migration are improved, meanwhile a user defined bandwidth is set, the minimum bandwidth is used as the migration bandwidth, and influence on normal I/O operation is reduced as much as possible.