Embodiments of the present invention provide a capacity expansion method and device, so as to reduce the time needed by capacity expansion of a hard disk. The method specifically includes: dividing, in a distributed redundant array of independent disks RAID system, each hard disk into virtual hard disks of equal size equally; dividing an original hard disk into X groups equally; selecting m virtual hard disks from each hard disk in sequence to form one original hard disk; selecting m virtual hard disks from each added hard disk and inserting the m virtual hard disks into the virtual hard disk group; moving, in the virtual hard disk group, data in virtual hard disks of the original hard disk to the virtual hard disks of the added hard disk. The present invention is applied to capacity expansion of a hard disk.