The invention discloses a staged power generation system by salt difference energy. The staged power generation system comprises a multistage osmosis device serially formed by a plurality of osmosis devices and a plurality of water turbines, each osmosis device comprises osmosis stages, the osmosis stage is formed by a plurality of osmosis membrane elements in parallel and used for dividing solutions different in concentration into a high-concentration side and a low-concentration side, the high-concentration side of each osmosis device corresponds to a water turbine, and the water turbines are coaxially connected to be used for driving a generator to operate; after a high-solubility solution and a low-solubility solution are respectively fed into the high-concentration side and the low-concentration side of each osmosis stage respectively, the osmosis stage can convert osmotic pressure difference into high-concentration side hydrostatic pressure, and the water turbines can be driven to rotate to drive the generator to operate by the aid of corresponding-size back pressure exerted to the osmosis stages. The invention further discloses a staged power generation method by the salt difference energy. Since different back pressure is exerted to different stages, system output power is enabled to be close to the maximum value, and power density of membranes is increased. Meanwhile, the osmotic membranes are fully utilized.