The invention relates to a preparation method of an indoor fireproof door. The technical problems that a preparation method of an existing fireproof door is poor in flame retardant effect, the method is inconvenient for automatic production, and the standard fireproof time cannot be reached are solved. The indoor fireproof door comprises a door frame and a door leaf, a door leaf framework, a fireproof expansion sealing piece and a perlite fireproof door core plate are sequentially arranged on the cross section of the door leaf, a gap is formed between the door frame and the door leaf, and the door frame and the door leaf are connected through fireproof hinges. The preparation method comprises the following steps that firstly, multiple layers of wood veneers and a glass magnesium board are pressed on a cold press through fireproof glue; the pressed large board is cut and sawed into battens; the battens are erected, assembled, stacked and framed; and the perlite fireproof plates are placed in the battens, gaps are filled with the fireproof sealing rubber strips, and the fireproof expansion strips play a fireproof role. The method can be applied to the field of preparation of indoor fireproof doors.