The invention discloses a method for preparing mixed rare earth oxides by a herbaceous plant precipitating agent. The method comprises the following steps: mixed chlorinated rare earth solution with a concentration of 0.1-2 mol/L is added in a reaction kettle for heating to reach a temperature of 20-100 DEG C; the herbaceous plant precipitating agent is added inside; the ratio of the liquid mass of the precipitating agent to the solute in the mixed chlorinated rare earth solution is 5-50: 1; when the pH value of a system to be reacted is 6-8, crystal precipitates are obtained; the precipitates and mother liquor are stood and aged for 0-72 hours at room temperature; and the precipitates are roasted by 1-5 h at 800-1200 DEG C after filtration and washing to obtain mixed rare earth oxide products. The waste water of the precipitates and the mother liquor contains no ammonia nitrogen, so that the effects of greenness and environmental protection are achieved; the process controllability is high; and the method is suitable for industrial production.