The invention relates to a preparation method of a polymer/graphene composite material through in situ reduction, which is characterized by comprising the following steps: adopting ultrasonic wave or grinding to evenly disperse the graphite oxide prepared by a Hummers method into polymer dispersion; introducing reducing agent into the polymer dispersion for in situ reduction, enabling the graphite oxide to be reduced into the grapheme so as to obtain stable polymer/graphene composite emulsion; carrying out demulsification, agglomeration and drying to obtain the composite polymer/grapheme composite master batch; adding the dried polymer/grapheme composite master batch and various assistants into the polymeric matrix according to a certain ratio; and carrying out double-roller mixing, vulcanization, melt extrusion or injection molding to obtain the polymer/graphene composite material with excellent physical and mechanical properties.