A casing including a first body and a locking plate is provided, the locking plate is installed to the first body, and the locking plate extends inward to form a frame, so as to lock the casing to a main chassis of a desktop computer. Compared with the existing technique, the casing is directly locked to the main chassis of the desktop computer from exterior to interior through the locking plate, i.e. the casing is adapted to install the redundant power supply without changing an internal configuration of the main chassis or changing an outline dimension of the main chassis or the casing. Moreover, in terms of material purchase, it is unnecessary to purchase casings of other dimensions, but only one type of casings are used to produce general power supplies and redundant power supplies, so as to effectively save the manufacturing cost. Moreover, a redundant power supply is provided.