The invention discloses a modular assembling method for a prefabricated spherical fragment of an ammunition warhead. The method comprises the following steps that modularization of the prefabricated spherical fragment is conducted, specifically, according to the technical requirements for the size, quantity, dimension, quality and center of mass of the fragment, the structural requirements of an inner cavity of a warhead are met, the shape and dimension of the fragment are designed, and the fragment is vertically and horizontally divided into a plurality of small pieces, a mold is designed according to the shape and dimension of each piece; according to the design weight of each piece of fragment, the pieces of fragments are weighed and put into the mold by adopting a regular arrangement method or a vibration method as the putting method; and after the pieces of fragments are put into the mold, an adhesive is poured into the mold, gaps among the fragments are filled with the adhesive,and a module with a required shape is formed after curing, so that the fragments and the mold are modularized into a whole. Compared with the prior art, module dividing can be performed according to the shape of an inner cavity of the warhead, the fragment modules with the needed shapes are obtained through the mold, the fragment modules are bonded to the corresponding positions of the inner cavity of the warhead, and the assembling requirements of fragments of various warheads are met.