The invention relates to a flame-retardant and antibacterial PET fiber and a slice spinning preparation method thereof. A functional masterbatch is prepared by melt blending of a PET slice and a metal-modified hyperbranched polymer, and then the functional masterbatch and the PET slice are mixed evenly and subjected to melt spinning to produce the flame-retardant and antibacterial PET fiber. The metal-modified hyperbranched polymer is evenly dispersed in the prepared product; the metal-modified hyperbranched polymer is a network polymer formed by crosslinking a hyperbranched polymer having theend groups containing carboxyl with metal ions, has insoluble and non-melt characteristics, is insoluble in most organic solvents at the temperature of 80 DEG C or less, and is non-melt in a range ofroom temperature to T, wherein T is more than or equal to 370 DEG C; cross-linking is achieved through linking oxygen atoms on double bonds in hyperbranched polymer molecules having the end groups containing carboxyl with the metal ions through coordination bonds and linking acid radical ions at the ends of the hyperbranched polymer molecules having the end groups containing carboxyl with metal ions by ionic bonds. The method is simple, and the prepared product has good antibacterial, flame retardant and mechanical properties.