Abstract: A flocculating technique was studied in which aluminium polychloride and polyacrylamide were used for flocculating treatment of black liquor from NH4OH-KOH pulping of rice straw in alkaline condition. The effects of flocculating conditions, such as dosage of 10% aluminium polychloride, dosage of 0. 1% polyacrylamide, reaction temperature and pH of black liquor on flocculation process were studied systematically by experiment to obtain suitable technological conditions. The results indicated that the rate of COD removal was 68% and the rate of lignin removal was 92% under the flocculating conditions. It was confirmed by IR and other analytic apparatus that 23. 73% N(dry basis), 6. 24% K(dry basis) and lots of lignin were present in the flocculation residue, so potentially it is a good solid fertilizer. The pH of supernatant was 9. 20 and the content of deleterious impurities in supernatant was low, and it could be recycled as cooking liquor. It helps to realize clean production of NH4OH-KOH pulping of rice straw.