The invention discloses a flexible net expanding device which is arranged on a trawl net of a fishing ship. The device comprises flexible water power floaters and flexible water power sinkers, wherein the flexible water power floaters and the flexible water power sinkers, which are flexible sheet-like objects, are connected in series to a floater line and a sinker line respectively; a plurality of shaping ropes are arranged on the flexible water power floaters and the flexible water power sinkers respectively; the shaping ropes on all the flexible water power floaters are connected in parallel; the shaping ropes on all the flexible water power sinkers are connected in parallel; the fishing ship pulls a fishing gear to move relative to water, the flexible water power floaters and the flexible water power sinkers form a concave fluid facing surface; the fluid facing surface faces the fishing ship; penetrating fluid through holes are formed in the flexible water power floaters and the flexible water power sinkers respectively. The flexible net expanding device has the advantages that the net expanding effect and the using energy-saving effect of the trawl net can be improved; the safety performance and the environmental protection performance of the trawl net are improved.