Ultrafiltration treatment of the ammonia sulfite pulping black liquor of wheat straw and its effects on chemical structure characteristics and physical properties were studied. The results showed that after uhrafiltration the content of lignin in retentate increased by 17.42% and the content of ash decreased by 18.22%. It was seen that the molecular weight distribution of lignin in black liquor and retentate were determined by Gel Chromatography. The molecular weight of lignin in permeate mainly concentrated in the range less than 4 950 with the occupation of 82. 86%, and the molecular weight of lignin in retentate was higher than that in black liquor. The contents of sulfonic, carboxyl, phenolic hydroxyl groups which are the main functional groups in retentate were all more than those in black liquor. Meanwhile, after uhrafihration, the retentate had better performance than black liquor as surfactant, as additive in concrete and as dispersant. Compared with black liquor, its surface tension at 1% was 54. 7 mN/m, and dropped 2.2 mN/m. Clean solution’ s fluidity improved from 70ram to 102ram. However, the performance of retentate was limited by uhrafihration so that it would be further improved by modification.