The lignin was extracted and purified by organic acid from pulping black liquor, and the process and influencing factors of extraction and purification were studied. The results of single factor experiments showed that the citric acid which contains three -COOH was suitable to extract and purify lignin. The results of orthogonal experiment demonstrated that the optimal condition of extraction and purification was as follows: citric acid saturated solution, 80°C, pH value of acid evolution endpoint was 3.0. The analysis of FT-IR spectrum, GPC and DSC indicated that the type of lignin extracted by citric acid was G-S, and the lignin was characterized as complete structure, higher molecular weight and Tg. The study on extraction technology of high purity lignin by citric acid method is hopeful to overcome the disadvantages of exorbitant cost and secondary pollution caused by the residues from lignin separation. It could change waste into valuables and put resource utilization of pulping black liquor into practice.