A low-bias-field method was proposed to form single multi-branched domains (MBDs) in garnet bubble films. The formation of MBDs is related to the nucleation of vertical Bloch lines (VBL) in their domain walls. With the decrease of the static bias-field Hb from the critical static bias field for multi-branched expansion Hd, the corresponding MBDs formed become more and more complex, accompanying the appearance of several kinds of hard domains. The fractal geometry was introduced into the study of MBDs. THe curved and branched patterns patterns of MBDs have been quantitatively described by the line-measuring dimension Dline, which is associated with the nucleation of VBL in their domain walls.