A data migration method for a storage system after expansion and a storage system are provided. After an ith expansion is performed on the storage system, data migration is performed by using an auxiliary balanced incomplete block design. Because a quantity of tuples including any element in the auxiliary balanced incomplete block design is identical, and each migration unit includes an identical quantity of parity chunks, a data migration amount after the expansion is minimized. In this way, time required for data migration after the expansion is significantly reduced, and a delay in a response to a user request that is caused because a data migration operation needs to be performed after the expansion is also reduced.