NOVELTY - Plant sewage treatment agent is obtained by pyrolyzing and dry distilling plant raw material at 450-1050 degrees C, fractionating and separating the obtained pyrolyzed dry distilled gas at 650-500 degrees C, separating out wood tar and water gas, removing the wood tar, condensing and separating the wood gas at room temperature, obtaining plant extraction raw solution, filtering, purifying and separating the plant extraction raw solution through active carbon layer, or standing and separating for at least 60 days to obtain refined plant sewage treatment agent.    USE - The plant sewage treatment agent is useful for treating sewage (claimed).    ADVANTAGE - The plant sewage treatment agent is highly-effective, has wide source and large selectivity, has no toxicity, does not cause pollution, cost-effective, can accelerate metabolic activities of microbes, can eliminate odor produced in sewage treatment process and avoid influence to surrounding environment. The organic matters in sewage is degraded effectively, and sewage quality and sewage treatment efficiency are improved, hence consequent sewage treatment load is reduced and sewage treatment time in unit volume is reduced by 10-25%. The water inlet quality of sewage treatment plant is improved. The sewage treatment operation cost is reduced by 15%.    DETAILED DESCRIPTION - Plant sewage treatment agent is obtained by using plant as raw material, pyrolyzing and dry distilling the plant raw material at 450-1050 degrees C, obtaining pyrolyzed dry distilled gas, fractionating and separating the pyrolyzed dry distilled gas at 650-500 degrees C, separating out wood tar and water gas, removing the wood tar, condensing and separating the wood gas at room temperature, obtaining plant extraction raw solution, filtering, purifying and separating the plant extraction raw solution through active carbon layer, or standing and separating for at least 60 days to obtain refined plant sewage treatment agent. An INDEPENDENT CLAIM is included for a method for treating sewage using the plant sewage treatment agent, which involves feeding the refined plant sewage treatment agent in sewage inlet of the general sewage treatment device in terms of sewage having chemical oxygen demand (COD) more than 8.5 mg/l, feeding 0.25-2.5 kg in each ton of sewage, and feeding refined plant sewage treatment agent that is diluted 100-200 times into sewage, and then agitating for 30-40 minutes, or in terms of sewage having COD less than 8.5 mg/l, feeding 0.25-2.5 kg in each ton of sewage, and feeding refined plant sewage treatment agent that is diluted 300-500 times into sewage, and then agitating for 30-40 minutes.