A method for preparing a glass piece for iron alloy XRF analysis based on a demould aid. The method is characterized in that: blending according to a mass ratio of ammonium iodide to demould aid to lithium carbonate being 1: (3 to 5): 30, and mixing to obtain a pre-oxidation flux containing the demould aid. 1.510 g of the pre-oxidation flux containing the demould aid is mixed with 0.2000 g of an iron alloy powder to obtain a mixture. The mixture is placed in a platinum crucible lined with lithium tetraborate, placed in a high temperature furnace, pre-oxidized at 680-720 DEG C for 15-20 minutes. The pre-oxidized platinum crucible is taken out, and then transferred to another high temperature furnace. After melting at 980 to 1050 DEG C for 15 to 25 minutes, the platinum crucible that the mixture is molten is taken out, naturally cooled, peeled off, and demolded to obtain the glass piece for iron alloy XRF analysis under the action of the demould aid. According to the invention, the method has the characteristics of environmental friendliness, low cost and short cycle, and the prepared glass piece for iron alloy XRF analysis has high analysis accuracy.