A preparation method of the fluorescent powder layer (108, 208) is provided. The said preparation method is carried out as following steps: step 1, a fluorescent powder layer (102, 202) is coated on one surface of a flat transparent substrate (101, 201); step 2, the surface of a board (103, 203) is shaped into a curved surface (104, 204); step 3, the board (103, 203) of step 2 is arranged on the substrate (101, 201) coated with the fluorescent powder layer (102, 202) of step 1; the curved surface (104, 204) of the board (103, 203) faces the substrate (101, 201), and the fluorescent powder layer (102, 202) is sandwiched between the substrate (101, 201) and the board (103, 203); step 4, the substrate (101, 201) is softened by heating, a pressure is then applied to the board (103, 203) so that the fluorescent powder layer (102, 202) sandwiched between the substrate (101, 201) and the board (103, 203) forms the fluorescent powder layer (108, 208) having the same curved shape as the curved surface (104, 204) of the board (103, 203) by the pressure, and the fluorescent powder layer (108, 208) on the substrate (101, 201) is completed after cooling.