A rubber reinforcing filler and a method for preparing rubber reinforcing filler by activation and modification of coal gangue are provided. The method includes the following steps: providing coal gangue raw material; crushing coal gangue raw materials; the crushed coal gangue raw materials are ultrafine grinding to obtain coal gangue particles; coal gangue particles were obtained by air flow classification. Activation of coal gangue particles; the activated gangue particles were surface modified and ground to obtain the modified gangue particles. The invention relates to the field of comprehensive utilization of coal gangue and the technical field of rubber filler. The method is simple and effective, which can complete the blending modification of coupling agent and activated coal gangue particles for surface modification at low cost, so as to avoid the waste of materials in the traditional modification process and realize the preparation of high quality rubber reinforcing filler.