The invention discloses a method and device for testing the performance of a flat plate-type forward osmosis membrane. The device is in a cylindrical form and comprises a feed solution pond body, a seal ring (7), a draw solution pond body and a fixed clamp (8), wherein the feed solution pond body and the draw solution pond body are semi-cylinders of the cylinder of the device. According to the device, in a testing process, a feed solution and a draw solution are evenly distributed in corresponding membrane ponds, and a flow guide segment is arranged in each pond; in the testing process, the required forward osmosis membrane is small in area and suitable for various performance tests of the flat plate-type forward osmosis membrane; and the device is fast and time-saving in installation and simple and convenient to operate.