The invention provides an erasure code based distributed storage system capacity expansion and reduction method. Before capacity expansion or capacity reduction, a disk array is identified, the free space of each disk in the disk array is checked, and relevant parameters m and n are acquired; the quantity of retained, remapped and reconstructed old used strips/new used strips (OUS/NUS) in the disk array is computed; the retained, remapped and reconstructed OUSs/NUSs are processed concurrently; and all verifying blocks are modified. The method can support bi-directional expansion of the disk array, the overhead generated during the expansion process is minimized, and the expansible process of a verifying array is accelerated.