The invention discloses a preparation method of a side wall multi-point flange of a metal shaped pipe, and belongs to the technical field of material processing. The preparation method adopts a hydraulic expansion technology to flange, and comprises the following steps of firstly, sealing the metal shaped pipe, and reserving a pressurizing port; according to the shape of the metal shaped pipe, designing and manufacturing an expansion die; finally, utilizing hydraulic expansion equipment and the expansion die to expand and flange at the side wall of the metal shaped pipe. The preparation method has the advantages that the flanging at any direction can be performed, the one-time forming in a multi-point and multi-direction way is realized, the positioning is accurate, the wrinkle is avoided, and the success rate of flanging is high; the flanging with higher requirement can be completed under the complicated condition, and the method is suitable for the flanging of irregular pipes with shaped cross section and space shape, and the flanging with stricter requirement on the flanging direction and requiring multi-point small distance.