An apparatus for osmotic power generation and desalination of seawater using a salinity difference is provided.The apparatus for osmotic power generation and desalination using a salinity difference includes: a first osmotic membrane reactor having a first salt water position space and a third salt water position space separated by a first forward osmotic membrane; a second osmotic membrane reactor having a second salt water position space and a draw solution position space separated by a second forward osmotic membrane; a high pressure pump connected between the second salt water position space and the third salt water position space and supplying salt water which has passed through the second salt water position space to the third salt water position space and pressurizing salt water, which has passed through the second salt water position space, to supply it to the third salt water position space such that pressure retarded osmosis can be made in the first osmotic membrane reactor; a desalination unit obtaining fresh water by separating a draw solute from a draw solution diluted through a transmission of water in salt water of the second salt water position space by way of the draw solution position space; and a turbine driven by flow force of salt water discharged from the third salt water position space to produce electric energy.