The invention discloses a reverse-osmosis seawater desalting device based on organic rankine cycle; the reverse-osmosis seawater desalting device comprises an organic rankine cycle system and a reverse-osmosis system; the organic rankine cycle system comprises an evaporator, an expansion machine, a condenser and a working medium pump communicated in sequence; the outlet of the working medium pump is communicated with the inlet of the evaporator; the reverse-osmosis system comprises a high-pressure pump, an energy recovery device, a reheater and a reverse-osmosis membrane connected in sequence; fresh water generated by the reverse-osmosis membrane enters a fresh water tank to be stored; strong brine is discharged into a strong brine tank; a waste heat supply pipeline is communicated with a waste heat pipeline of the reheater through a branched pipeline; an electric adjusting valve is arranged on the branched pipeline; and the evaporator, the condenser and the reheater are convective heat transfer devices. According to the invention, industrial waste heat is used as the power in the reverse-osmosis seawater desalting process, so that high-efficiency recovery of waste heat is realized; simultaneously, raw seawater is pre-heated to the optimal temperature of the reverse-osmosis membrane, so that the reverse-osmosis membrane always runs at the optimal temperature; and thus, the service life of the reverse-osmosis membrane is effectively prolonged.