The invention discloses a fixed-position welding device used for a range hood fan shell and a welding method thereof. The device comprises a supporting table, a clamping assembly and a mould assembly.The clamping assembly comprises a clamping press block, an X-direction regulating mechanism, a Y-direction regulating mechanism and a Z-direction regulating mechanism. The X-direction regulating mechanism comprises a rodless cylinder. The Y-direction regulating mechanism comprises an adjusting screw rod and an adjusting hand wheel, the adjusting hand wheel is located at the top end of the adjusting screw rod, a slide block is arranged on the adjusting screw rod, and the clamping press block is hinged and fixed to the slide block. The Z-direction regulating mechanism comprises an adjusting cylinder and a fixing support. The mould assembly comprises an arc-shaped mould, a fixing sleeve and a fixing rod. The welding method comprises the following steps that (1) the mould assembly is mounted;and (2) the fan shell is welded and machined. The fixed-position welding device is ingenious and reasonable in structural design, by pressing and positioning sheets, the attaching levelness between every two adjacent sheets is guaranteed, accordingly, the welding quality of subsequent sheets is improved, the welding precision is improved, the quality of the fan shell is improved, and the servicelife of the fan shell is prolonged.