Abstract: A noval process, based on the combination of CaO and polyaluminium chloride (PAC) flocculation, was developed to selectively remove lignin from pre-hydrolysis liquor (PHL). The selective removals of lignin by using acidification, alkalization and combined process were investigated and the mechanism of removal lignin by PAC was investigated. The results showed that the maximum removal rates (poplar chip 15.68% and wheat straw 18.76%) of acidification were obtained at pH value of 1.5 and the removal rates of CaO treatment reached the maximum (poplar chip 33.33% and wheat straw 30.67%) as adjusting the pH value with CaO to 12. After the CaO treatment for the hydrolysate, both the removal rate and selectivity of lignin were higher than those of acidification treatment. Using CaO to adjust pH to 10 and then adding PAC to 36 mmol/L, the removal rate of lignin in pre-hydrolysis liquor reached the maximum, the numbers were 50.59% and 49.17% for poplar chips and wheat straw, respectively. Meanwhile, the selectivites were the best, represents 86.57% and 82.76% for poplar chips and wheat straw, respectively.