The invention discloses a double-inlet and double-outlet plate heat exchanger and a seawater desalting device. The double-inlet and double-outlet plate heat exchanger comprises a front end plate, a heat dissipating net, a brazing flux layer, a back end plate, a first heat exchange plate, a flow control valve, a temperature sensor and a third water inlet; the front end plate is provided with a first water inlet; the flow control valve is arranged in the first water inlet; a second water inlet is formed beside the first water inlet; the third water inlet is formed beside the second water inlet;and fastening bolts are arranged on two sides of the third water inlet. The double-inlet and double-outlet plate heat exchanger has the following beneficial effects: the original heat exchange effectcan be improved; the seawater flow is increased; the seawater flowing speed is lowered; the pipe scouring is reduced; the equipment service life is prolonged; seawater can be fed from double sides ofthe front and back ends, so that the joint flowing speed of the plate heat exchanger of a seawater system is reduced, and the heat exchange effect is improved; as one more joint is provided, the totalseawater flow is increased under the same pressure; and through double-inlet and double-outlet, the pipe circulating capacity is improved, the joint flowing speed is reduced, and the wear is reduced.