The invention relates to a preparation method of a middle wedge-shaped resonant cavity layer of a linear gradual filter. The preparation method comprises the following steps: arranging a baffle with a triangular window between an ion beam emission window and a sample piece to be machined in a direction vertical to the emission direction of an ion beam; overlapping the triangular top with one end of the sample piece along the emission direction of the ion beam; when the ion beam is etched, moving the sample piece at a constant speed back and forth; after etching for certain times, and obtaining thickness difference in a direction vertical to a sample movement direction to obtain a designed wedge-shaped separation layer. Before the ion beam is etched, the etching speed of a middle-layer medium material needs to be calibrated and a suitable material etching speed is obtained by adjusting parameters of etching the ion beam in a certain range. Compared with an existing preparation method of a wedge-shaped resonant cavity layer cavity, the preparation method has the advantages of simple and easy manufacturing process, simplicity in process operation and high yield, film thickness is more accurately controlled and the batch production can be realized and the like; the production cost of a mini-type spectrograph taking the linear gradual filter as a core light splitting assembly is reduced.