The invention discloses a method for cultivating a synchronous nitrogen and sulfur removal mixed flora. The method comprises the steps that anaerobic ammonium oxidation granular sludge is inoculated in a reactor, simulated wastewater containing ammonia nitrogen, nitrate nitrogen, sulfide, inorganic salt, trace elements and sodium bicarbonate is taken as influent water, the temperature is 30-36 DEGC, and the reactor runs to be stable under the anaerobic condition of controlling the hydraulic retention time to be constant; the cultivation of the synchronous nitrogen and sulfur removal mixed flora is completed by means of a method of increasing the concentration of the sulfide in the influent water gradually, decreasing the concentration of the ammonia nitrogen and nitrite gradually, then increasing the concentration of the sulfide gradually and finally shortening the hydraulic retention time in sequence. According to the method, the anaerobic ammonium oxidation sludge is adopted as a single inoculum, the synchronous nitrogen and sulfur removal mixed flora is cultivated by means of the step-by-step domestication method, and the treatment of the wastewater simultaneously containing the ammonia nitrogen, nitrate nitrogen and sulfide in chemical plants and the like is facilitated.