A fixing device includes a gasket and a clip. The clip includes a clip upper portion, a clip body portion, and a clip leg portion. The clip upper portion has a width wider than the clip body portion and the clip leg portion. The clip upper portion, the clip body portion, and the clip leg portion are connected to form an open loop facing downward. The gasket includes a gasket upper portion, a gasket intermediate portion, and a gasket base portion forming an “A” shape. The gasket is compressible. The clip upper portion and the clip body portion define two receiving areas evenly distributed at two inner sides of the clip. The gasket is evenly and simultaneously compressed toward the two receiving areas of the clip while the clip moves downward without tilting or slanting.