a fuel supplying system of a liquid propane injection engine , may include a fuel tank for storing fuel , a main fuel pump connected to the fuel tank and disposed at the exterior of the fuel tank , a fuel supply line connecting the main fuel pump and the engine to supply fuel from the main fuel pump to the engine , and a fuel return line connecting the engine and the interior of the fuel tank to return fuel from the engine to the interior of the fuel tank , wherein a sub - fuel pump is provided to the interior of the fuel tank and connected to the fuel supply line through a sub - fuel pump connecting pipe , the sub - fuel pump being operated when a predetermined condition is satisfied .