The invention relates to a preparation method of ABS polybutadiene rubber latex with small grain diameter, comprising components based on a proportioning rate by weight: butadiene 34.00-45.00 percent, secondary monomer 0.01-6.0 percent, thiol 0.12-0.22 percent, emulsion 0.80-2.60 percent, electrolyte 0.12-0.32 percent and desalted water, wherein, the emulsion is made by mixing potassium rosinate and potassium oleate; the electrolyte comprises sodium pyrophosphate or EDTA and potassium carbonate. The emulsion is added in two steps, the temperature is controlled based on a saddle-shaped temperature controlling curve with 63-69 DEG C at prophase, 60-64 DEG C at metaphase and 70-75 DEG C at anaphase, the reaction time is 7-9 hours under 0.8 MPa, or reaction speed at the prophase is accelerated to discharge more heat, then the reaction temperature is decrease when the conversion rate is at 40 percent-80 percent, the heat discharging speed at the metaphase is decreased, then the reaction temperature is raised to a highest point when the conversion rate is more than 80 percent, so as to change a violent heat discharging situation and realize in shortening the reaction time. Quadric adding of the emulsion is used, so as to realize the effects of concentration increasing and reaction pressure decreasing to stably control the polymerization of the polybutadiene, thereby reducing rubber evolution and adhesion of the rubber latex and the conversion rate is more than 97 percent.