Bamboo pulp black liquor modified by graft sulfonation（GCL1-JB） was fractionated into four fractions with different relative molecular mass（RMM） by ultrafiltration,and the RMM distributions of GCL1-JB fractions were characterized by means of GPC.Then,the effects of GCL1-JB fractions on the performances of fresh cement paste and mortar were analyzed.The results show that（1） the fraction with high RMM is of better water-reducing dispersion performance;（2） when the dosage of the fraction with high RMM（more than 50000） is 0.5%,the fluidity of cement paste reaches 287mm at a water to cement ratio of 0.29,the fluidity loss ratio reaches 7% after 120min,and the 3-day,7-day and 28-day compressive strength ratios of mortar are respectively up to 159.4%,193.4% and 143.8%;（3） the fraction with moderate RMM is of strong air-entraining and retarding effect,which improves the workability of fresh mortar,although the long-term compressive strength of hardened mortar is relative low;and（4） when the dosage of the fraction with moderate RMM（10000～50000） is 0.5%,the initial setting time and final setting time of cement paste prolong for 140 and 297min,respectively,and the 28-day compressive strength ratio of hardened mortar is up to 99.8%.